U.S. patent number 10,689,319 [Application Number 16/165,575] was granted by the patent office on 2020-06-23 for processes for purifying acetic acid and hydrating anhydride.
This patent grant is currently assigned to CELANESE INTERNATIONAL CORPORATION. The grantee listed for this patent is CELANESE INTERNATIONAL CORPORATION. Invention is credited to Yaw-Hwa Liu, Mark O. Scates, Ronald D. Shaver.
United States Patent |
10,689,319 |
Shaver , et al. |
June 23, 2020 |
Processes for purifying acetic acid and hydrating anhydride
Abstract
Processes for purifying acetic acid by distilling a process
stream in a column in which acetic anhydride is formed in the lower
portion of the column. The product stream withdrawn from the column
comprises acetic acid, water at a concentration of no more than 0.2
wt. %, and acetic anhydride at a concentration of no more than 600
wppm. The process further comprises hydrating the acetic anhydride
in the product stream to form a purified acetic acid product
comprising acetic anhydride at a concentration of no more than 50
wppm.
Inventors: |
Shaver; Ronald D. (Houston,
TX), Liu; Yaw-Hwa (Missouri City, TX), Scates; Mark
O. (Houston, TX) |
Applicant: |
Name |
City |
State |
Country |
Type |
CELANESE INTERNATIONAL CORPORATION |
Irving |
TX |
US |
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Assignee: |
CELANESE INTERNATIONAL
CORPORATION (Irving, TX)
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Family
ID: |
58692007 |
Appl.
No.: |
16/165,575 |
Filed: |
October 19, 2018 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20190055184 A1 |
Feb 21, 2019 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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15863484 |
Jan 5, 2018 |
10160714 |
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15348466 |
Mar 6, 2018 |
9908835 |
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62255060 |
Nov 13, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C07C
51/56 (20130101); C07C 51/47 (20130101); C07C
51/087 (20130101); C07C 51/487 (20130101); C07C
51/44 (20130101); C07C 51/12 (20130101); C07C
51/087 (20130101); C07C 53/08 (20130101); C07C
51/47 (20130101); C07C 53/08 (20130101); C07C
51/56 (20130101); C07C 53/12 (20130101); C07C
51/44 (20130101); C07C 53/08 (20130101); C07C
51/487 (20130101); C07C 53/08 (20130101); C07C
51/12 (20130101); C07C 53/08 (20130101) |
Current International
Class: |
C07C
51/44 (20060101); C07C 51/487 (20060101); C07C
51/12 (20060101); C07C 51/47 (20060101); C07C
51/56 (20060101); C07C 51/087 (20060101) |
Field of
Search: |
;562/519 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0161874 |
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Jul 1992 |
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EP |
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721301 |
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Jan 1955 |
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GB |
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2112394 |
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Jul 1982 |
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GB |
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H0867650 |
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Mar 1996 |
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JP |
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H10231267 |
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Sep 1998 |
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JP |
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H11315046 |
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Nov 1999 |
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JP |
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2001505199 |
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Apr 2001 |
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JP |
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Other References
Celanese Chemicals, Product Description, "Acetic Acid, Glacial
(Ethanoic Acid, Methanecarboxylic Acid)", Acetic Acid, Chemical
Abstracts Registry, No. 67-19-7, 2000. cited by applicant .
Celanese, "Acetic Acid", Celanese--The chemistry inside
innovation.TM., 2013. Brochure. cited by applicant .
International Search Report received in the related International
Patent Application No. PCT/US2016/061570, dated Jan. 31, 2017.
cited by applicant .
Title--"Control of propionic acid content in acetic acid production
by carbonylation of methanol", Aug. 25, 2013, 20, pp. 50-52, p. 51,
right column, line 4-13. cited by applicant .
Title--"Control of formation of ethanol in methanol", Sep. 15,
2007, 5.sup.th period, pp. 21-22, left column, lines 8-16. cited by
applicant .
Title--"A method to reduce ethanol content in purified methanol",
Aug. 25, 2010, vol. 33, No. 4, pp. 225-227, p. 225, left column,
line 2-9. cited by applicant .
Title--"Purification of crude methanol", Jan. 15, 1997, 1997,
1.sup.st period, pp. 1-5 and 11, p. 2, left column, line 24-right
column, line 3. cited by applicant .
Kirk-Othmer Encyclopedia of Chemical Technology 4.sup.th Ed, Mass
Transfer to Neuroregulators, A Wiley-Interscience publication; John
Wiley & Sons, Inc., vol. 16, 1995, p. 554, lines 9-17. cited by
applicant .
IMPCA Methanol Reference Specifications, International Methanol
Producers & Consumers Association, Dec. 9, 2010, p. 1. cited by
applicant .
Title: "Specification of ethanol content of methanol products", May
15, 2008, 2008, 3.sup.rd period, pp. 52-54, p. 53, left column,
lines 14-20. cited by applicant .
Title: "Consideration for four-column distillation process for
methanol production", Sep. 20, 1998, 1998, 9.sup.th period, pp.
21-22. cited by applicant .
Knopp, et al., "The Thermodynamics of the Thermal Decomposition of
Acetic Acid in the Liquid Phase", J. Phys. Chem, 1962, pp.
1513-1516. cited by applicant.
|
Primary Examiner: Carr; Deborah D
Attorney, Agent or Firm: Kilpatrick Townsend & Stockton
LLP
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
This patent application is a continuation of U.S. application Ser.
No. 15/863,484, filed Jan. 5, 2018, which is a continuation of U.S.
application Ser. No. 15/348,466, filed on Nov. 10, 2016, now U.S.
Pat. No. 9,908,835, which claims the priority of U.S. Application
No. 62/255,060, filed Nov. 13, 2015, the disclosure of which is
incorporated herein by reference in its entirety.
Claims
We claim:
1. A process for producing acetic acid comprising: a.
carbonylating, in a reactor, at least one member selected from the
group consisting of methanol, dimethyl ether, and methyl acetate in
a reaction medium comprising water at a concentration from 0.1 to
14 wt. %, a rhodium catalyst, methyl iodide, and an iodide salt; b.
separating the reaction medium to form a liquid recycle stream and
a vapor product stream; c. distilling at least a portion of the
vapor product stream in a first column to obtain an overhead and a
sidestream and withdrawing the side stream from a location of the
first column having more than 0.2 wt. % water; i. biphasically
separating the overhead to form a light phase comprising water and
a heavy phase comprising methyl iodide and at least one PRC; and
ii. treating the heavy phase to remove the at least one PRC; d.
distilling the side stream in a second column to obtain a product
stream and withdrawing the product stream from a location of the
second column having less than 0.2 wt. % water; and e. contacting
the product stream with a cation exchange resin to form a purified
acetic acid product.
2. The process of claim 1, wherein the purified acetic product
comprises no more than 50 wppm acetic anhydride.
3. The process of claim 1, wherein the product stream is hydrated
in the cation exchange resin.
4. The process of claim 3, wherein the hydration in the cation
exchange resin reduces the acetic anhydride in the product stream
by at least 60%.
5. The process of claim 3, wherein the product stream has a flow
rate through the cation exchange resin ranging from 0.1 bed volumes
per hour to 50 bed volumes per hour.
6. The process of claim 3, wherein the cation exchange resin
comprises sulfonic acid or partially metal impregnated sulfonic
acid functional groups.
7. The process of claim 3, wherein the product stream is not
contacted with an aqueous stream prior to contacting cation
exchange resin.
8. The process of claim 3, wherein the cation exchange resin
comprises a strong acid macroreticular or macroporous resin.
9. The process of claim 3, wherein the cation exchange resin
comprises a chelating resin or zeolite.
10. The process of claim 3, wherein the product stream is contacted
with the cation exchange resin in a vessel made of a
corrosion-resistant metal.
11. The process of claim 1, further comprising contacting the
purified acetic acid product with a metal-exchanged ion exchange
resin having acid cation exchange sites to reduce the iodide
concentration of the purified acetic acid product.
12. The process of claim 1, wherein the product stream comprises
acetic anhydride at a concentration from 5 to 600 wppm.
13. The process of claim 1, wherein the product stream further
comprises lithium in a concentration of up to or equal to 10
wppm.
14. The process of claim 1, wherein the product stream further
comprises iodide in a concentration from 20 wppb to 1.5 wppm.
15. The process of claim 1, wherein the acetic anhydride in the
product stream is at a concentration from 10 to 600 wppm.
16. The process of claim 1, wherein the location in the first
column is above a feed of the vapor product stream.
17. The process of claim 1, vapor product stream comprises water in
an amount from 0.5 to 14 wt. %.
18. The process of claim 1, further comprising refluxing a
condensed portion of the overhead stream to the first column.
19. The process of claim 1, wherein the water concentration in the
overhead stream is from 5 to 80 wt %.
20. The process of claim 1, wherein the location in the second
column is near the bottom of the second column.
Description
FIELD OF THE INVENTION
This invention relates to processes for producing acetic acid and,
in particular, to improved processes for hydrating acetic anhydride
in a product stream to form a purified acetic acid product
comprising acetic anhydride at a concentration of no more than 50
wppm.
BACKGROUND OF THE INVENTION
Among currently employed processes for synthesizing acetic acid,
one of the most useful commercially is the catalyzed carbonylation
of methanol with carbon monoxide as taught in U.S. Pat. No.
3,769,329, incorporated herein by reference in its entirety. The
carbonylation catalyst contains rhodium, either dissolved or
otherwise dispersed in a liquid reaction medium or supported on an
inert solid, along with a halogen-containing catalyst promoter as
exemplified by methyl iodide. The rhodium can be introduced into
the reaction system in any of many forms. Likewise, because the
nature of the halide promoter is not generally critical, a large
number of suitable promoters, most of which are organic iodides,
may be used. Most typically and usefully, the reaction is conducted
by continuously bubbling carbon monoxide gas through a liquid
reaction medium in which the catalyst is dissolved.
A widely used and successful commercial process for synthesizing
acetic acid involves the catalyzed carbonylation of methanol with
carbon monoxide. The catalyst contains rhodium and/or iridium and a
halogen promoter, typically methyl iodide. The reaction is
conducted by continuously bubbling carbon monoxide through a liquid
reaction medium in which the catalyst is dissolved. The reaction
medium comprises acetic acid, methyl acetate, water, methyl iodide
and the catalyst. Commercial processes for the carbonylation of
methanol include those described in U.S. Pat. No. 3,769,329.
Another conventional methanol carbonylation process includes the
Cativa.TM. process, which is discussed in Jones, J. H. (2002), "The
Cativa.TM. Process for the Manufacture of Acetic Acid," Platinum
Metals Review, 44 (3): 94-105, the entirety of which is
incorporated herein by reference.
The AO.TM. process for the carbonylation of an alcohol to produce
the carboxylic acid having one carbon atom more than the alcohol in
the presence of a rhodium catalyst is disclosed in U.S. Pat. Nos.
5,001,259; 5,026,908; and 5,144,068; and EP0161874, the entireties
of which are incorporated herein by reference. As disclosed
therein, acetic acid is produced from methanol in a reaction medium
containing methyl acetate (MeAc), methyl halide, especially methyl
iodide (MeI), and rhodium present in a catalytically effective
concentration. These patents disclose that catalyst stability and
the productivity of the carbonylation reactor can be maintained at
high levels, even at very low water concentrations, i.e., 4 weight
percent or less, (despite the prior practice of maintaining
approximately 14-15 wt. % water) by maintaining in the reaction
medium, along with a catalytically effective amount of rhodium, at
least a finite concentration of water, e.g., 0.1 wt. %, and a
specified concentration of iodide ions over and above the iodide
ion that is present as hydrogen iodide. This iodide ion is a simple
salt, with lithium iodide being preferred. The salt may be formed
in situ, for example, by adding lithium acetate, lithium carbonate,
lithium hydroxide or other lithium salts of anions compatible with
the reaction medium. The patents teach that the concentration of
methyl acetate and iodide salts are significant parameters in
affecting the rate of carbonylation of methanol to produce acetic
acid, especially at low reactor water concentrations. By using
relatively high concentrations of the methyl acetate and iodide
salt, a high degree of catalyst stability and reactor productivity
is achieved even when the liquid reaction medium contains water in
finite concentrations as low as 0.1 wt. %. Furthermore, the
reaction medium employed improves the stability of the rhodium
catalyst, i.e., resistance to catalyst precipitation, especially
during the product recovery steps of the process. In these steps,
distillation for the purpose of recovering the acetic acid product
tends to remove from the catalyst the carbon monoxide, which in the
environment maintained in the reaction vessel, is a ligand with
stabilizing effect on the rhodium.
U.S. Pat. No. 5,144,068 discloses a process for producing acetic
acid by reacting methanol with carbon monoxide in a liquid reaction
medium containing a rhodium (Rh) catalyst and comprising water,
acetic acid, methyl iodide, and methyl acetate, wherein catalyst
stability is maintained in the reaction by maintaining in said
reaction medium during the course of said reaction 0.1 wt. % to 14
wt. % of water together with (a) an effective amount in the range
of 2 wt. % to 20 wt. % of a catalyst stabilizer selected from the
group consisting of iodide salts which are soluble in said reaction
medium in effective concentration at reaction temperature, (b) 5
wt. % to 20 wt. % of methyl iodide, and (c) 0.5 wt. % to 30 wt. %
of methyl acetate. Suitable iodide salts may be a quaternary iodide
salt or an iodide salt of a member of the group consisting of the
metals of Group IA and Group IIA of the Periodic Table.
Carbonyl impurities, such as acetaldehyde, that are formed during
the carbonylation of methanol may react with iodide catalyst
promoters to form multi-carbon alkyl iodides, e.g., ethyl iodide,
propyl iodide, butyl iodide, pentyl iodide, hexyl iodide, and the
like. It is desirable to remove multi-carbon alkyl iodides from the
reaction product because even small amounts of these impurities in
the acetic acid product tend to poison the catalyst used in the
production of vinyl acetate, a product commonly produced from
acetic acid.
Conventional techniques to remove such impurities include treating
the crude acid product streams with oxidizers, ozone, water,
methanol, activated-carbon, amines, and the like. Such treatments
may or may not be combined with distillation of the acetic acid.
The most typical purification treatment involves a series of
distillations to yield a suitable purified acetic acid as the final
product. It is also known to remove carbonyl impurities from
organic streams by treating the organic streams with an amine
compound such as hydroxylamine, which reacts with the carbonyl
compounds to form oximes, followed by distillation to separate the
purified organic product from the oxime reaction products. However,
the additional treatment of the purified acetic acid adds cost to
the process, and distillation of the treated acetic acid product
can result in additional impurities being formed.
While it is possible to obtain acetic acid of relatively high
purity, the acetic acid product formed by the low-water
carbonylation process and purification treatment described above
frequently remains somewhat deficient with respect to the
permanganate time due to the presence of small proportions of
residual impurities. Because a sufficient permanganate time is an
important commercial test, which the acid product may be required
to meet to be suitable for many uses, the presence of impurities
that decrease permanganate time is objectionable. Moreover, it has
not been economically or commercially feasible to remove minute
quantities of these impurities from the acetic acid by distillation
because some of the impurities have boiling points close to that of
the acetic acid product or halogen-containing catalyst promoters,
such as methyl iodide. It has thus become important to identify
economically viable methods of removing impurities elsewhere in the
carbonylation process without contaminating the purified acetic
acid or adding unnecessary costs.
Macroreticulated or macroporous strong acid cationic exchange resin
compositions are conventionally utilized to reduce iodide
contamination. Suitable exchange resin compositions, e.g., the
individual beads thereof, comprise both sites that are
functionalized with a metal, e.g., silver, mercury or palladium,
and sites that remain in the acid form. Exchange resin compositions
that have little or no metal-functionality do not efficiently
remove iodides and, as such, are not conventionally used to do so.
Typically, metal-functionalized exchange resins are provided in a
fixed bed and a stream comprising the crude acetic acid product is
passed through the fixed resin bed. In the metal functionalized
resin bed, the iodide contaminants contained in the crude acetic
acid product are removed from the crude acid product stream.
Widely used and successful commercial processes for synthesizing
acetic anhydride also involves the catalyzed carbonylation of
methanol with carbon monoxide. Acetic anhydride processes have been
disclosed in U.S. Pat. Nos. 5,292,948; 4,374,070; 4,115,444; and
4,046,807, the entireties of which are incorporated herein by
reference.
Other ion exchange resins have been used to remove iodide
impurities from acetic acid and/or acetic anhydride. U.S. Pat. No.
6,657,078 describes a low-water process that uses a
metal-functionalized exchange resin to remove iodides. The
reference also avoids the use of a heavy ends column, resulting in
energy savings. U.S. Pat. No. 5,220,058 also discloses the use of
ion exchange resins having metal exchanged thiol functional groups
for removing iodide impurities from acetic acid and/or acetic
anhydride. Typically, the thiol functionality of the ion exchange
resin has been exchanged with silver, palladium, or mercury. U.S.
Pat. No. 5,227,524 discloses a process for removing iodide
derivatives from liquid acetic acid and/or acetic anhydride
comprises contacting the liquid acetic acid and/or acetic anhydride
with a strong acid cation exchange resin having from about 4% to
about 12% crosslinking, a surface area in the proton exchanged form
of less than 10 m.sup.2 g.sup.-1 after drying from the water wet
state and a surface area of greater than 10 m.sup.2 g.sup.-1 after
drying from a wet state in which water has been replaced by
methanol. The resin has at least one percent of its active sites
converted to the silver form, preferably from 30 to 70 percent.
U.S. Pat. No. 5,801,279 discloses a method which can reduce the
amount of silver or mercury dissolved in a solution after contact
and can increase the usage of silver or mercury without installing
new treating facilities in a process for removing iodine compounds
contained in an organic medium, particularly acetic acid or a
mixture of acetic acid or acetic anhydride, by contacting them with
a cation exchange resin in which at least 1% of the active sites
are converted to a silver form or a mercury form. This disclosed
method is characterized by carrying out the operation while
elevating the temperatures in stages while contacting the organic
medium, particularly acetic acid or a mixture of acetic acid and
acetic anhydride, containing the iodine compounds with a cation
exchange resin.
U.S. Pat. No. 5,344,976 discloses that the metal ion contaminants
in the acid and/or anhydride may arise from corrosion or the use of
reagents in the upstream process. The patent describes the use of a
cationic exchanger in the acid form to remove at least a portion of
the metal ion contaminants such as iron, potassium, calcium,
magnesium, and sodium from a carboxylic acid stream prior to
contacting the stream with the exchanged strong acid cation
exchange resin to remove C.sub.1 to C.sub.10 alkyl iodide
compounds, hydrogen iodide or iodide salts.
U.S. Pat. No. 5,648,531 discloses a process for continuously
producing acetic anhydride alone or acetic anhydride and acetic
acid by reacting methyl acetate and/or dimethyl ether and,
optionally, water and/or methanol, with carbon monoxide alone or
carbon monoxide and hydrogen in the presence of a rhodium compound
and methyl iodide as principal catalysts. Trace impurities
causative of tar formation are distilled and separated in an
evaporator and/or a subsequent refining step to remove the same.
The removal of the trace impurities causative of tar formation
serves to decrease the amount of tar formed as an impurity.
U.S. Pat. No. 8,759,576 discloses a process for purifying acetic
anhydride. The process includes the steps of feeding a liquid crude
acetic anhydride stream directly to a distillation column and
separating the liquid crude acetic anhydride stream to produce a
light ends stream, a sidedraw and a residue stream. The sidedraw
comprises substantially pure acetic anhydride product. The
distillation column is operated at a pressure less than 101 kPa.
The substantially pure acetic anhydride product comprises greater
than 98 wt. % acetic anhydride, has a permanganate time of greater
than 10 minutes, and has an APHA color of less than 10.
While the above-described processes have been successful, the need
exists for improved processes for producing acetic acid, in
particular, for methods for removing acetic anhydride from those
processes.
SUMMARY OF THE INVENTION
This invention generally relates to processes for the production of
acetic acid. One embodiment of the present invention relates to a
process for purifying acetic acid, comprising distilling a process
stream in a column, the process stream comprising acetic acid at a
concentration of greater than 90 wt. %, water at a concentration
from 1 to 3 wt. %, one or more C.sub.1-C.sub.14 alkyl iodides in a
total concentration of no more than 6 wt. %, and methyl acetate at
a concentration of no more than 6 wt. %, forming acetic anhydride
in a lower portion of the column, withdrawing a product stream from
the lower portion of the column, the product stream comprising
acetic acid, water at a concentration of no more than 0.2 wt. %,
and acetic anhydride at a concentration of no more than 600 wppm,
e.g., from 5 to 600 wppm, and hydrating the acetic anhydride in the
product stream to form a purified acetic acid product comprising
acetic anhydride at a concentration of no more than 50 wppm, e.g.,
from 0.5 to 50 wppm. Preferably, the acetic anhydride concentration
in the purified acetic acid product is less than the acetic
anhydride concentration in the product stream. In one embodiment,
the water concentration is maintained at a concentration of no more
than 0.2 wt. %. The hydrating step may reduce the acetic anhydride
concentration by at least 60%. In one embodiment, the hydrating
step comprises contacting the product stream with a cation exchange
resin. The cation exchange resin may comprise sulfonic acid or
partially metal impregnated sulfonic acid functional groups. In one
embodiment, the product stream is not contacted with an aqueous
stream after withdrawal of the product stream from the lower
portion of the column. The process stream that is distilled is
substantially free of acetic anhydride, e.g., the acetic anhydride
concentrations are below detectable limits. In addition to acetic
acid and water, the process stream may also comprise hydrogen
iodide at a concentration of no more than 300 wppm. The product
stream may be withdrawn at a point within 5 actual stages from the
base of the column. The water concentration in the lower portion of
the column is maintained to be less than the water concentration of
the process stream. In further embodiments, the process comprises
contacting the purified acetic acid product with a metal-exchanged
ion exchange resin having acid cation exchange sites to reduce the
iodide concentration of the purified acetic acid product to no more
than 100 wppb.
In another embodiment, there is provided a process for purifying
acetic acid comprising carbonylating, in a reactor, at least one
member selected from the group consisting of methanol, dimethyl
ether, and methyl acetate in a reaction medium comprising water at
a concentration from 0.1 to 14 wt. %, a rhodium catalyst, methyl
iodide, and an iodide salt, separating the reaction medium to form
a liquid recycle stream and a vapor product stream, distilling at
least a portion of the vapor product stream in a first column to
obtain a side stream comprising acetic acid at a concentration
greater than 90 wt. %, water at a concentration from 1 to 3 wt. %,
one or more C.sub.1-C.sub.14 alkyl iodides in a total concentration
of no more than 6 wt. % and methyl acetate at a concentration of no
more than 6 wt. %, distilling the side stream in a second column to
obtain a product stream comprising acetic acid, water at a
concentration of no more than 0.2 wt. % and acetic anhydride at a
concentration of no more than 600 wppm, e.g., from 10 to 600 wppm,
and contacting the product stream with a cation exchange resin to
form a purified acetic acid product comprising no more than 50 wppm
acetic anhydride. In one embodiment, the production rate of acetic
anhydride is greater in the second column than the reactor. The
side stream is substantially free of acetic anhydride, e.g., the
acetic anhydride concentrations are below detectable limits. The
side stream may also comprise hydrogen iodide at a concentration of
no more than 300 wppm. The purified acetic acid product may
comprise water at a concentration of no more than 0.2 wt. % and
acetic anhydride at a concentration of no more than 10 wppm. In one
embodiment, the water concentration in a lower portion of the
second column is maintained to be less than the water concentration
of the side stream. The acetic anhydride concentration of the
purified acetic acid product is less than the acetic anhydride
concentration of the product stream. The cation exchange resin may
reduce the acetic anhydride concentration of the product stream by
at least 60%. In further embodiments, the process comprises
contacting the purified acetic acid product with a metal-exchanged
ion exchange resin having acid cation exchange sites to reduce the
iodide concentration of the purified acetic acid product to no more
than 100 wppb.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be better understood in view of the
appended non-limiting figures, wherein:
FIG. 1 illustrates a process for purifying acetic acid with cation
exchange resin to hydrate acetic anhydride and a metal
functionalized fixed resin bed for iodide removal.
FIG. 2 illustrates another process for purifying acetic acid from a
sidedraw with cation exchange resin to hydrate acetic anhydride and
a metal functionalized fixed resin bed for iodide removal.
DETAILED DESCRIPTION OF THE INVENTION
At the outset, it should be noted that in the development of any
such actual embodiment, numerous implementation-specific decisions
must be made to achieve the developer's specific goals, such as
compliance with system related and business related constraints,
which will vary from one implementation to another. In addition,
the processes disclosed herein can also comprise components other
than those cited or specifically referred to, as is apparent to one
having average or reasonable skill in the art.
In the summary and this detailed description, each numerical value
should be read once as modified by the term "about" (unless already
expressly so modified), and then read again as not so modified
unless otherwise indicated in context. Also, in the summary and
this detailed description, it should be understood that a
concentration range listed or described as being useful, suitable,
or the like, is intended that any and every concentration within
the range, including the end points, is to be considered as having
been stated. For example, a range "from 1 to 10" is to be read as
indicating each and every possible number along the continuum
between about 1 and about 10. Thus, even if specific data points
within the range, or even no data points within the range, are
explicitly identified or refer to only a few specific data points,
it is to be understood that inventors appreciate and understand
that any and all data points within the range are to be considered
to have been specified, and that inventors possessed knowledge of
the entire range and all points within the range.
Throughout the entire specification, including the claims, the
following terms have the indicated meanings unless otherwise
specified.
As used in the specification and claims, "near" is inclusive of
"at." The term "and/or" refers to both the inclusive "and" case and
the exclusive "or" case, and is used herein for brevity. For
example, a mixture comprising acetic acid and/or methyl acetate may
comprise acetic acid alone, methyl acetate alone, or both acetic
acid and methyl acetate.
All percentages are expressed as weight percent (wt. %), based on
the total weight of the particular stream or composition present,
unless otherwise noted. Room temperature is 25.degree. C. and
atmospheric pressure is 101.325 kPa unless otherwise noted.
For purposes herein: acetic acid may be abbreviated as "AcOH";
acetaldehyde may be abbreviated as "AcH"; methyl acetate may be
abbreviated "MeAc"; methanol may be abbreviated "MeOH"; methyl
iodide may be abbreviated as "MeI"; hydrogen iodide may be
abbreviated as "HI"; carbon monoxide may be abbreviated "CO"; and
dimethyl ether may be abbreviated "DME".
HI refers to either molecular hydrogen iodide or dissociated
hydriodic acid when at least partially ionized in a polar medium,
typically a medium comprising at least some water. Unless otherwise
specified, the two are referred to interchangeably. Unless
otherwise specified, HI concentration is determined via acid-base
titration using a potentiometric end point. In particular, HI
concentration is determined via titration with a standard lithium
acetate solution to a potentiometric end point. It is to be
understood that for purposes herein, the concentration of HI is not
determined by subtracting a concentration of iodide assumed to be
associated with a measurement of corrosion metals or other non H+
cations from the total ionic iodide present in a sample.
It is to be understood that HI concentration does not refer to
iodide ion concentration. HI concentration specifically refers to
HI concentration as determined via potentiometric titration.
This subtraction method is an unreliable and imprecise method to
determine relatively lower HI concentrations (i.e., less than about
5 weight percent) due to the fact that it assumes all non-H+
cations (such as cations of Fe, Ni, Cr, Mo) are associated with
iodide anion exclusively. In reality, a significant portion of the
metal cations in this process can be associated with acetate anion.
Additionally, many of these metal cations have multiple valence
states, which adds even more unreliability to the assumption on the
amount of iodide anion which could be associated with these metals.
Ultimately, this method gives rise to an unreliable determination
of the actual HI concentration, especially in view of the ability
to perform a simple titration directly representative of the HI
concentration.
For purposes herein, an "overhead" or "distillate" of a
distillation column refers to at least one of the lower boiling
condensable fractions which exits at or near the top, (e.g.,
proximate to the top), of the distillation column, and/or the
condensed form of that stream or composition. Obviously, all
fractions are ultimately condensable, yet for purposes herein, a
condensable fraction is condensable under the conditions present in
the process as readily understood by one of skill in the art.
Examples of noncondensable fractions may include nitrogen,
hydrogen, and the like. Likewise, an overhead stream may be taken
just below the upper most exit of a distillation column, for
example, wherein the lowest boiling fraction is a non-condensable
stream or represents a de-minimis stream, as would be readily
understood by one of reasonable skill in the art.
The "bottoms" or "residuum" of a distillation column refers to one
or more of the highest boiling fractions which exit at or near the
bottom of the distillation column, also referred to herein as
flowing from the bottom sump of the column. It is to be understood
that a residuum may be taken from just above the very bottom exit
of a distillation column, for example, wherein the very bottom
fraction produced by the column is a salt, an unusable tar, a solid
waste product, or a de-minimis stream as would be readily
understood by one of reasonable skill in the art.
For purposes herein, distillation columns comprise a distillation
zone and a bottom sump zone. The distillation zone includes
everything above the bottom sump zone, i.e., between the bottom
sump zone and the top of the column. For purposes herein, the
bottom sump zone refers to the lower portion of the distillation
column in which a liquid reservoir of the higher boiling components
is present (e.g., the bottom of a distillation column) from which
the bottom or residuum stream flows upon exiting the column. The
bottom sump zone may include reboilers, control equipment, and the
like.
It is to be understood that the term "passages," "flow paths,"
"flow conduits," and the like in relation to internal components of
a distillation column are used interchangeably to refer to holes,
tubes, channels, slits, drains, and the like, which are disposed
through and/or which provide a path for liquid and/or vapor to move
from one side of the internal component to the other side of the
internal component. Examples of passages disposed through a
structure such as a liquid distributor of a distillation column
include drain holes, drain tubes, drain slits, and the like, which
allow a liquid to flow through the structure from one side to
another.
Average residence time is defined as the sum total of all liquid
volume hold-up for a given phase within a distillation zone divided
by the average flow rate of that phase through the distillation
zone. The hold-up volume for a given phase can include liquid
volume contained in the various internal components of the column
including collectors, distributors and the like, as well as liquid
contained on trays, within downcomers, and/or within structured or
random packed bed sections.
Hydration of Acetic Anhydride
The present invention relates to processes for the purification of
acetic acid and, in particular, to improved processes for hydrating
acetic anhydride in a product stream comprising acetic acid, water
at a concentration of no more than 0.2 wt. %, and acetic anhydride
at a concentration of no more than 600 wppm. Advantageously, the
disclosed embodiments provide a purified acetic acid product
comprising acetic anhydride at a concentration of no more than 50
wppm, e.g., no more than 40 wppm, no more than 30 wppm, no more
than 20 wppm, no more than 10 wppm or no more than 5 wppm. In terms
of ranges, the purified acetic acid product comprises acetic
anhydride at a concentration from 0.5 to 50 wppm, e.g., from 0.5 to
40 wppm, from 0.5 to 30 wppm, from 0.5 to 20 wppm, or from 0.5 to
10 wppm. The purified acetic acid product comprises less acetic
anhydride than the product stream. This process yields a high
quality purified acetic acid product that may be widely used in
various applications.
During purification of acetic acid obtained by methanol
carbonylation, one or more distillation columns may be used to
separate impurities from the process stream and yield a purified
product stream according to embodiments of the present invention.
Typically, in one of the distillation columns, water is separated
to yield a glacial acetic acid product having a water concentration
of no more than 0.2 wt. %, e.g., no more than 0.15 wt. %, no more
than 0.1 wt. %, or no more than 0.05 wt. %. It has been found that
in the lower portion of this column, due to the substantially
anhydrous conditions (no more than 0.2 wt. % water), acetic
anhydride side reactions may occur, leading to acetic anhydride
formation that contaminates the product stream. This may lead to
deleterious acetic acid products that have quality control issues.
Although acetic acid/acetic anhydride co-production may involve
separating these components, it is not practical or efficient to
separate acetic anhydride from the product stream using such
methods. Advantageously, the present invention overcomes problems
associated with the acetic anhydride contamination by hydrating the
product to reduce and/or eliminate acetic anhydride.
In one embodiment, the process stream fed to the distillation
columns is substantially free of acetic anhydride, meaning the
acetic anhydride concentrations are below detectable limits.
Detectable limits may detect acetic anhydride in concentration of
greater than 0.5 wppm. Stated differently, the process stream is
obtained from a methanol carbonylation process that produces acetic
acid under aqueous conditions, e.g., in a reaction medium
comprising water at a concentration of greater than 0.1 wt. %.
Thus, the process stream is substantially free of acetic anhydride
produced in the carbonylation reactor.
In one embodiment, there is provided a process for purifying acetic
acid, comprising distilling a process stream in a column, the
process stream comprising acetic acid and water, wherein the
process stream is substantially free of acetic anhydride, forming
acetic anhydride in a lower portion of the column, withdrawing a
product stream from the lower portion of the column, the product
stream comprising acetic acid, water at a concentration of no more
than 0.2 wt. %, and acetic anhydride at a concentration of no more
than 600 wppm, and hydrating the acetic anhydride in the product
stream to form a purified acetic acid product comprising acetic
anhydride at a concentration of no more than 50 wppm.
The process streams that are separated comprises acetic acid,
water, and other components, such as, but not limited to methyl
iodide, methyl acetate, hydrogen iodide, acetaldehyde, and
propionic acid. Various process streams are disclosed herein. One
exemplary process stream comprises acetic acid in amount of greater
than or equal to 90 wt. %, e.g., greater than or equal to 94 wt. %
or greater than or equal to 96 wt. %. The water concentration of
the process stream may be in an amount from 1 to 3 wt. %, e.g.,
preferably from 1 to 2.5 wt. % and more preferably from 1.1 to 2.1
wt. %. The process stream may also comprise one or more
C.sub.1-C.sub.14 alkyl iodides in a total concentration of no more
than 6 wt. %, e.g., no more than 4 wt. %, or no more than 3.6 wt.
%, and methyl acetate at a concentration of no more than 6 wt. %,
e.g., no more than 4 wt. %, or no more than 3.6 wt. %. In some
embodiments, in addition to acetic acid and water, the exemplary
process stream may also comprise one or more C.sub.1-C.sub.14 alkyl
iodides in an amount from 0.1 to 6 wt. %, e.g., from 0.5 to 5 wt.
%, from 0.6 to 4 wt. %, from 0.7 to 3.7 wt. %, or from 0.8 to 3.6
wt. %. Generally, methyl iodide is the primary alkyl iodide and the
concentration of the one or more C.sub.1-C.sub.14 alkyl iodides may
be determined by the methyl iodide concentration. Due to the
presence of water, the process stream may also contain methyl
acetate in an amount from 0.1 to 6 wt. %, e.g., from 0.5 to 5 wt.
%, from 0.6 to 4 wt. %, from 0.7 to 3.7 wt. %, or from 0.8 to 3.6
wt. %. In some embodiments, the process stream may also comprise
hydrogen iodide at a concentration of no more than 300 wppm, e.g.,
or no more than 250 wppm, no more than 200 wppm, no more than 100
wppm, no more than 50 wppm, no more than 25 wppm, or no more than
10 wppm.
In one embodiment, there is provided a process for purifying acetic
acid comprising distilling a process stream in a column, the
process stream comprising acetic acid at a concentration greater
than 90 wt. %, water at a concentration from 1 to 3 wt. %, one or
more C.sub.1-C.sub.14 alkyl iodides in a total concentration of no
more than 6 wt. % (from 0.1 to 6 wt. %), and methyl acetate at a
concentration of no more than 6 wt. % (from 0.1 to 6 wt. %), and
optionally hydrogen iodide at a concentration of no more than 300
wppm, forming acetic anhydride in a lower portion of the column,
withdrawing a product stream from the lower portion of the column,
the product stream comprising acetic acid, water at a concentration
of no more than 0.2 wt. %, and acetic anhydride at a concentration
of no more than 600 wppm, and hydrating the acetic anhydride in the
product stream to form a purified acetic acid product comprising
acetic anhydride at a concentration of no more than 50 wppm.
In one embodiment, the distillation column separates a process
stream to yield a product stream comprising acetic acid, water, and
acetic anhydride. The presence of acetic anhydride in the product
stream is undesirable. In one embodiment, the product stream
comprises acetic acid at a concentration of greater than or equal
to 99.5 wt. %, e.g., greater than or equal to 99.7 wt. % or greater
than or equal to 99.9 wt. %. The product stream comprises water at
a concentration of no more than 0.2 wt. %, e.g., no more than 0.15
wt. %, no more than 0.1 wt. %, or no more than 0.05 wt. %. The
product stream comprises a concentration of acetic anhydride that
is undesirable and causes product quality issues. Even low amounts
of acetic anhydride, e.g., amounts of 5 wppm, may contribute to
product quality issues. In one embodiment, the product stream
comprises acetic anhydride at a concentration of no more than 600
wppm, e.g., no more than 500 wppm, no more than 450 wppm, no more
than 400 wppm, no more than 300 wppm, no more than 200 wppm, no
more than 100 wppm, or no more than 50 wppm. In terms of ranges,
the product stream comprises acetic anhydride in an amount from 5
to 600 wppm, e.g., from 10 to 600 wppm, from 5 to 450 wppm, from 10
to 450 wppm, from 10 to 300 wppm, or from 10 to 100 wppm. The
embodiments of the present invention may also be used to reduce
acetic anhydride in the product stream when the concentration
exceeds 600 wppm. To treat these higher concentrations of acetic
anhydride, the hydration may be repeated one or more times. Of
course, if acetic anhydride is introduced into the column, the
acetic anhydride may be higher than when it is not added, and the
present invention may also remove the added acetic anhydride.
The product stream may be withdrawn from a location of a
distillation column that yields a product stream having a water
concentration that is less than the water concentration in the
process stream. It has been found that in this portion of the
distillation column, that even if no acetic anhydride is introduced
into the column, acetic anhydride may be undesirably formed in the
presence of substantially anhydrous conditions, e.g. no more than
0.2 wt. %. For example, the product stream may be withdrawn from
the base of a distillation column or as a sidedraw from a lower
portion of the distillation. In one embodiment, the sidedraw is
withdrawn within 5 actual stages of the base, e.g., within 4 actual
stages, within 3 actual stages, within 2 actual stages, within 1
actual stage, and preferably above the base of the second column.
The second column may have from 10 to 80 actual states, e.g., from
15 to 80 actual stages or from 20 to 80 actual stages. An actual
stage may correspond to a plate in a column. The sidedraw may be a
liquid or vapor sidedraw.
In one embodiment, there is provided a process for purifying acetic
acid, comprising distilling a process stream in a column, the
process stream comprising acetic acid and water, wherein the
process stream is substantially free of acetic anhydride, forming
acetic anhydride in a lower portion of the column, withdrawing a
product stream from the lower portion of the column, the product
stream comprising acetic acid, water at a concentration of no more
than 0.2 wt. %, and acetic anhydride at a concentration of no more
than 600 wppm, and contacting the product stream with a cation
exchange resin to hydrate the acetic anhydride in the product
stream to form a purified acetic acid product comprising acetic
anhydride at a concentration of no more than 50 wppm.
In one embodiment, the hydration of acetic anhydride may be
conducted by contacting the product stream with a cation exchange
resin. The product stream may be withdrawn from the distillation
column contacted and with the cation exchange resin. Acetic
anhydride is reacted in the cation exchange resin through a
hydration reaction to convert the acetic anhydride into acetic
acid. Because water has already been removed from the product
stream, it is not desirable to increase the water concentration in
the cation exchange resin. Thus, no aqueous streams are contacted
with the product stream after withdrawal from the lower portion. In
one embodiment, the water concentration in the cation exchange
resin is maintained at or below the water concentration of the
product stream, e.g., a water concentration of no more than 0.2 wt.
%, e.g., no more than 0.15 wt. %, no more than 0.1 wt. %, or no
more than 0.05 wt. %.
Suitable cation exchange resins for the hydration of acetic
anhydride may comprise strong acid cation exchange resins, for
example strong acid macroreticular or macroporous resins, for
example Amberlyst.RTM. 15 resin (DOW), Purolite C145, or Purolite
CT145. In one embodiment, the cation exchange resin comprises
sulfonic acid or partially metal impregnated sulfonic acid
functional groups. The resin may also be an acid-form strong acid
cation exchange mesoporous resin. Chelating resins and zeolites may
also be used.
Advantageously, by hydrating the product stream in the cation
exchange resin, the acetic anhydride of the product stream may be
reduced by at least 60%, e.g., at least 70%, at least 75%, at least
80%, or at least 85%. In one embodiment, the process for purifying
acetic acid comprises distilling a process stream in a column, the
process stream comprising acetic acid and water, forming acetic
anhydride in a lower portion of the column, withdrawing a product
stream from the lower portion, wherein the product stream comprises
acetic anhydride, and hydrating the at least 60% of the acetic
anhydride in the product stream to form a purified acetic acid
product.
As disclosed further herein, the product stream may contain iodide
in a concentration from 20 wppb to 1.5 wppm. The iodide may be
removed by a metal ion-exchange resin. The metal-exchanged ion
exchange resin can have at least 1% of the strong acid exchange
sites occupied by silver, mercury, palladium, and/or rhodium, e.g.,
at least 2% silver, mercury, palladium, and/or rhodium, at least 5%
silver, mercury, palladium, and/or rhodium, at least 10% silver,
mercury, palladium, and/or rhodium, or at least 20% silver,
mercury, palladium, and/or rhodium. The product stream have may an
iodide content of greater than 100 wppb, e.g., greater than 100
wppb, greater than 200 wppb, greater than 400 wppb, greater than
500 wppb, or greater than 1000 wppb, prior to treatment with the
metal ion-exchange resin and an iodide content of less than 10
wppb, e.g., less than 10 wppb, less than 7 wppb, less than 5 wppb,
less than 3 wppb, less than 2 wppb, less than 1 wppb, after
contacting the resin.
Cation Removal
Accordingly in one embodiment, the cation exchange resin for
hydrating acetic anhydride is positioned upstream of the metal
ion-exchange resin for removing iodides. This configuration may be
desirable when, the cation exchange resin may also remove metal ion
contaminants, in particular lithium cations, that cause
displacement of the metals of the ion exchange resin. In one
embodiment, the lithium cation in the product stream to be removed
using the cation exchange resin may be derived from and/or
generated by a compound in the reaction medium. In some
embodiments, the cation exchange resin may remove cations may be
selected from the group consisting of Groups IA and IIA of the
periodic table, quaternary nitrogen cations, and
phosphorous-containing cations. Higher alkyl iodides,
C.sub.10-C.sub.14 alkyl iodides, may also be removed using the
cation exchange resin.
In one embodiment, the cation exchange resin may also reduce
lithium concentration in the product stream. Lithium has also been
found to be entrained in the crude acid product in the absence of
heavy ends and other finishing apparatus. Even very small amounts
of lithium in the product stream, e.g., 10 wppb, may cause problems
with removing iodides. Lithium may be present in the product stream
as one or more lithium-containing compounds such as lithium iodide,
lithium hydroxide, lithium acetate, lithium acetate dihydrate,
lithium carbonate, lithium alkyl carbonate, methyl lithium, lithium
chloride, or lithium oxalate. For purposes herein, the
concentration of lithium-containing compound is reported as the
concentration of the lithium in the lithium-containing compound.
The product stream may comprise lithium in a concentration of up to
or equal to 10 wppm, e.g., up to or equal to 5 wppm, up to or equal
to 1 wppm, up to or equal to 500 wppb, up to or equal to 300 wppb,
or up to or equal to 100 wppb. In terms of ranges, the crude acid
product may comprise lithium in an amount from 0.01 wppm to 10
wppm, e.g., from 0.05 wppm to 5 wppm or from 0.05 wppm to 1 wppm.
By utilizing a cationic exchanger in the acid form before
introducing the crude acid product to a metal-exchanged resin,
significant amounts of lithium can be removed. For example greater
than or equal to 90 wt. % of the lithium in the stream may be
removed by the cationic exchanger, e.g., greater than or equal to
92 wt. %, greater than or equal to 95 wt. %, greater than or equal
to 98 wt. %, or greater than or equal to 99 wt. %. Thus, the stream
exiting the acid-form cationic exchanger may contain no more than
50 wppb lithium, e.g., less than 25 wppb lithium, no more than 10
wppb, or no more than 5 wppb. Such removal of the lithium can
greatly extend the life of the metal-exchanged resin.
In other embodiments, the lithium concentration in the product
stream may be controlled by removing the product stream as a vapor
sidedraw from the distillation column. When a vapor sidedraw is
used, the product stream is condensed prior to contacting the
cation exchange resin to hydrate the acetic anhydride and,
optionally, remove cations.
In other embodiments, the cation exchange resin may be positioned
downstream of the metal ion-exchange resin for removing iodides, or
positioned in parallel with the metal ion-exchange resin for
treating a portion of the product stream.
Acetic Acid Production Systems
An exemplary acetic acid production process is described below. In
the interest of clarity, not all features of an actual
implementation are described in this specification. It will of
course be appreciated that in the development of any such actual
embodiment, numerous implementation-specific decisions must be made
to achieve the developers' specific goals, such as compliance with
system-related and business-related constraints, which will vary
from one implementation to another. Moreover, it will be
appreciated that such a development effort might be complex and
time-consuming, but would nevertheless be a routine undertaking for
those of ordinary skill in the art having the benefit of this
disclosure.
In one embodiment, there is provided a process for producing acetic
acid comprising carbonylating, in a reactor, at least one member
selected from the group consisting of methanol, dimethyl ether, and
methyl acetate in a reaction medium comprising water at a
concentration from 0.1 to 14 wt. %, a rhodium catalyst, methyl
iodide, and an iodide salt, separating the reaction medium to
obtain a process stream comprising acetic acid and water,
distilling the process stream in a column to obtain a product
stream comprising acetic acid, water at a concentration of no more
than 0.2 wt. % and acetic anhydride at a concentration of no more
than 600 wppm and contacting the product stream with a cation
exchange resin to form a purified acetic acid product comprising no
more than 50 wppm acetic anhydride.
The purification processes described herein may be useful in
carbonylation processes that use methanol and/or methyl acetate
(MeAc), methyl formate or dimethyl ether, or mixtures thereof, to
produce acetic acid in the presence of a Group VIII metal catalyst,
such as rhodium, and a halogen-containing catalyst promoter. A
particularly useful process is the low water rhodium-catalyzed
carbonylation of methanol to acetic acid as exemplified in U.S.
Pat. No. 5,001,259. Other metal catalysts, e.g., iridium-based
catalysts, are contemplated as well.
Generally, the metal component, e.g., rhodium component, of the
catalyst system is believed to be present in the form of a
coordination compound of rhodium with a halogen component providing
at least one of the ligands of such coordination compound. In
addition to the coordination of rhodium and halogen, it is also
believed that carbon monoxide coordinates with rhodium. The rhodium
component of the catalyst system may be provided by introducing
into the reaction zone rhodium in the form of rhodium metal,
rhodium salts such as the oxides, acetates, iodides, carbonates,
hydroxides, chlorides, etc., or other compounds that result in the
formation of a coordination compound of rhodium in the reaction
environment.
The metal catalyst may comprise a Group VIII metal. Suitable Group
VIII catalysts include rhodium and/or iridium catalysts. When a
rhodium catalyst is used, the rhodium catalyst may be added in any
suitable form such that rhodium is in the catalyst solution as an
equilibrium mixture including [Rh(CO).sub.2I.sub.2]-anion, as is
well known in the art. Iodide salts optionally maintained in the
reaction mixtures of the processes described herein may be in the
form of a soluble salt of an alkali metal or alkaline earth metal,
quaternary ammonium, phosphonium salt or mixtures thereof. In
certain embodiments, the catalyst co-promoter is lithium iodide,
lithium acetate, or mixtures thereof. The salt co-promoter may be
added as a non-iodide salt that generates an iodide salt. The
iodide catalyst stabilizer may be introduced directly into the
reaction system. Alternatively, the iodide salt may be generated
in-situ since under the operating conditions of the reaction
system, a wide range of non-iodide salt precursors reacts with
methyl iodide or hydroiodic acid in the reaction medium to generate
the corresponding co-promoter iodide salt stabilizer. For
additional detail regarding rhodium catalysis and iodide salt
generation, see U.S. Pat. Nos. 5,001,259; 5,026,908; 5,144,068 and
7,005,541, the entireties of which are hereby incorporated by
reference. The carbonylation of methanol utilizing iridium catalyst
is well known and is generally described in U.S. Pat. Nos.
5,942,460, 5,932,764, 5,883,295, 5,877,348, 5,877,347 and
5,696,284, the entireties of which are hereby incorporated by
reference.
The halogen-containing catalyst promoter of the catalyst system
consists of a halogen compound comprising an organic halide. Thus,
alkyl, aryl, and substituted alkyl or aryl halides can be used.
Preferably, the halogen-containing catalyst promoter is present in
the form of an alkyl halide. Even more preferably, the
halogen-containing catalyst promoter is present in the form of an
alkyl halide in which the alkyl radical corresponds to the alkyl
radical of the feed alcohol, which is being carbonylated. Thus, in
the carbonylation of methanol to acetic acid, the halide promoter
may include methyl halide, and more preferably methyl iodide.
The components of the reaction medium are maintained within defined
limits to ensure sufficient production of acetic acid as the
primary product and thus are not directed to co-production process
for producing acetic acid along with a secondary product, such as
acetic anhydride, in the reaction medium The reaction medium
contains a concentration of the metal catalyst, e.g., rhodium
catalyst, in an amount from 200 to 3000 wppm as rhodium, e.g., from
500 to 2000 wppm, or from 600 to 1500 wppm. The concentration of
water in the reaction medium is maintained under low water
conditions, e.g., water in amount of no more than 14 wt. %, from
0.1 wt. % to 14 wt. %, from 0.2 wt. % to 10 wt. % or most
preferably from 0.25 wt. % to 5 wt. %. The concentration of methyl
iodide in the reaction medium is maintained to be from 1 to 25 wt.
%, e.g., from 5 to 20 wt. %, from 4 to 13.9 wt. %. The
concentration of iodide salt, e.g., lithium iodide, in the reaction
medium is maintained to be from 1 to 25 wt. %, e.g., from 2 to 20
wt. %, from 3 to 20 wt. %. The concentration of methyl acetate in
the reaction medium is maintained to be from 0.5 to 30 wt. %, e.g.,
from 0.3 to 20 wt. %, from 0.6 to 4.1 wt. %. The concentration of
acetic acid in the reaction medium is generally greater than or
equal to 30 wt. %, e.g., greater than or equal to 40 wt. %, greater
than or equal to 50 wt. %, or greater than or equal to 60 wt. %.
Similar to the process stream disclosed herein, the reaction medium
is substantially free of acetic anhydride. The following amounts
are based on the total weight of the reaction medium.
In embodiments, the process for producing acetic acid further
includes introducing a lithium compound into the reactor to
maintain the concentration of lithium acetate in an amount from 0.3
to 0.7 wt. % in the reaction medium, wherein in an exemplary
embodiment, in the reaction medium the concentration of the rhodium
catalyst is maintained in an amount from 200 to 3000 wppm as
rhodium in the reaction medium, the concentration of water is
maintained in amount from 0.1 to 4.1 wt. % in the reaction medium,
and the concentration of methyl acetate is maintained from 0.6 to
4.1 wt. % in the reaction medium, based on the total weight of the
reaction medium present within the carbonylation reactor.
In embodiments, the lithium compound introduced into the reactor is
selected from the group consisting of lithium acetate, lithium
carboxylates, lithium carbonates, lithium hydroxide, other organic
lithium salts, and mixtures thereof. In embodiments, the lithium
compound is soluble in the reaction medium. In an embodiment,
lithium acetate dihydrate may be used as the source of the lithium
compound.
Lithium acetate reacts with hydrogen iodide according to the
following equilibrium reaction (I) to form lithium iodide and
acetic acid: LiOAc+HI.revreaction.LiI+HOAc (I)
Lithium acetate is thought to provide improved control of hydrogen
iodide concentration relative to other acetates, such as methyl
acetate, present in the reaction medium. Without being bound by
theory, lithium acetate is a conjugate base of acetic acid and thus
reactive toward hydrogen iodide via an acid-base reaction. This
property is thought to result in an equilibrium of the reaction (I)
which favors reaction products over and above that produced by the
corresponding equilibrium of methyl acetate and hydrogen iodide.
This improved equilibrium is favored by water concentrations of
less than 4.1 wt. % in the reaction medium. In addition, the
relatively low volatility of lithium acetate compared to methyl
acetate allows the lithium acetate to remain in the reaction medium
except for volatility losses and small amounts of entrainment into
the vapor crude product. In contrast, the relatively high
volatility of methyl acetate allows the material to distill into
the purification train, rendering methyl acetate more difficult to
control. Lithium acetate is much easier to maintain and control in
the process at consistent low concentrations of hydrogen iodide.
Accordingly, a relatively small amount of lithium acetate may be
employed relative to the amount of methyl acetate needed to control
hydrogen iodide concentrations in the reaction medium. It has
further been discovered that lithium acetate is at least three
times more effective than methyl acetate in promoting methyl iodide
oxidative addition to the rhodium [I] complex. However, it has been
discovered that lithium cations derived from and/or generated by
the lithium compound in the reaction medium may be entrained or be
volatile enough to concentrate with the crude acetic acid product
after purification in the primary purification trains.
In embodiments, the concentration of lithium acetate in the
reaction medium is maintained at greater than or equal to 0.3 wt.
%, or greater than or equal to 0.35 wt. %, or greater than or equal
to 0.4 wt. %, or greater than or equal to 0.45 wt. %, or greater
than or equal to 0.5 wt. %, and/or in embodiments, the
concentration of lithium acetate in the reaction medium is
maintained at no more than 0.7 wt. %, or no more than 0.65 wt. %,
or no more than 0.6 wt. %, or no more than 0.55 wt. %.
It has been discovered that an excess of lithium acetate in the
reaction medium can adversely affect the other compounds in the
reaction medium, leading to decrease productivity. Conversely, it
has been discovered that a lithium acetate concentration in the
reaction medium below about 0.3 wt. % is unable to maintain the
desired hydrogen iodide concentrations in the reaction medium of
below 1.3 wt. %.
In embodiments, the lithium compound may be introduced continuously
or intermittently into the reaction medium. In embodiments, the
lithium compound is introduced during reactor start up. In
embodiments, the lithium compound is introduced intermittently to
replace entrainment losses.
Thus, in one embodiment, there is provided a process for producing
acetic acid comprising carbonylating, in a reactor, at least one
member selected from the group consisting of methanol, dimethyl
ether, and methyl acetate in a reaction medium comprising water at
a concentration from 0.1 to 14 wt. %, a rhodium catalyst, methyl
iodide, an iodide salt, and lithium acetate, separating the
reaction medium to obtain a process stream comprising acetic acid
and water, distilling the process stream in a column to obtain a
product stream comprising acetic acid, water at a concentration of
no more than 0.2 wt. % and acetic anhydride at a concentration of
no more than 600 wppm and contacting the product stream with a
cation exchange resin to form a purified acetic acid product
comprising no more than 50 wppm acetic anhydride.
In some embodiments, the desired reaction rates are obtained even
at low water concentrations by maintaining in the reaction medium
an ester of the desired carboxylic acid and an alcohol, desirably
the alcohol used in the carbonylation, and an additional iodide ion
that is over and above the iodide ion that is present as hydrogen
iodide. A desired ester is methyl acetate. The additional iodide
ion is desirably an iodide salt, with lithium iodide (LiI) being
preferred. It has been found, as described in U.S. Pat. No.
5,001,259, that under low water concentrations, methyl acetate and
lithium iodide act as rate promoters.
The carbonylation reaction of methanol to acetic acid product may
be carried out by contacting the methanol feed with gaseous carbon
monoxide bubbled through an acetic acid solvent reaction medium
containing the rhodium catalyst, methyl iodide promoter, methyl
acetate, and additional soluble iodide salt, at conditions of
temperature and pressure suitable to form the carbonylation
product. It will be generally recognized that it is the
concentration of iodide ion in the catalyst system that is
important and not the cation associated with the iodide, and that
at a given molar concentration of iodide the nature of the cation
is not as significant as the effect of the iodide concentration.
Any metal iodide salt, or any iodide salt of any organic cation, or
other cations such as those based on amine or phosphine compounds
(optionally, ternary or quaternary cations), can be maintained in
the reaction medium provided that the salt is sufficiently soluble
in the reaction medium to provide the desired level of the iodide.
When the iodide is a metal salt, preferably it is an iodide salt of
a member of the group consisting of the metals of Group IA and
Group IIA of the periodic table as set forth in the "Handbook of
Chemistry and Physics" published by CRC Press, Cleveland, Ohio,
2002-03 (83rd edition). In particular, alkali metal iodides are
useful, with lithium iodide being particularly suitable. In the low
water carbonylation process, the additional iodide ion over and
above the iodide ion present as hydrogen iodide is generally
present in the catalyst solution in amounts such that the total
iodide ion concentration is from 1 to 25 wt. % and the methyl
acetate is generally present in amounts from 0.5 to 30 wt. %, and
the methyl iodide is generally present in amounts from 1 to 25 wt.
%. The rhodium catalyst is generally present in amounts from 200 to
3000 wppm as rhodium.
The reaction medium may also contain impurities that should be
controlled to avoid byproduct formation. One impurity in the
reaction medium may be ethyl iodide, which is difficult to separate
from acetic acid. Applicant has further discovered that the
formation of ethyl iodide may be affected by numerous variables,
including the concentration of acetaldehyde, ethyl acetate, methyl
acetate and methyl iodide in the reaction medium. Additionally,
ethanol content in the methanol source, hydrogen partial pressure
and hydrogen content in the carbon monoxide source have been
discovered to affect ethyl iodide concentration in the reaction
medium and, consequently, propionic acid concentration in the final
acetic acid product.
In embodiments, the propionic acid concentration in the acetic acid
product may further be maintained below 250 wppm by maintaining the
ethyl iodide concentration in the reaction medium at no more than
750 wppm without removing propionic acid from the acetic acid
product.
In embodiments, the ethyl iodide concentration in the reaction
medium and propionic acid in the acetic acid product may be present
in a weight ratio from 3:1 to 1:2. In embodiments, the
acetaldehyde:ethyl iodide concentration in the reaction medium is
maintained at a weight ratio from 2:1 to 20:1.
In embodiments, the ethyl iodide concentration in the reaction
medium may be maintained by controlling at least one of the
hydrogen partial pressure, the methyl acetate concentration, the
methyl iodide concentration, and/or the acetaldehyde concentration
in the reaction medium.
In embodiments, the concentration of ethyl iodide in the reaction
medium is maintained/controlled to be no more than 750 wppm, or
e.g., no more than 650 wppm, or no more than 550 wppm, or no more
than 450 wppm, or no more than 350 wppm. In embodiments, the
concentration of ethyl iodide in the reaction medium is
maintained/controlled at greater than or equal to 1 wppm, or e.g.,
5 wppm, or 10 wppm, or 20 wppm, or 25 wppm, and no more than 650
wppm, or e.g., 550 wppm, or 450 wppm, or 350 wppm.
In embodiments, the weight ratio of ethyl iodide in the reaction
medium to propionic acid in the acetic acid product may range from
3:1 to 1:2, or e.g., from 5:2 to 1:2, or from 2:1 to 1:2, or from
3:2 to 1:2.
In embodiments, the weight ratio of acetaldehyde to ethyl iodide in
the reaction medium may range from 20:1 to 2:1, or e.g., from 15:1
to 2:1, from 9:1 to 2:1, or from 6:1.
Carbonylation Reaction
Typical reaction temperatures for carbonylation may be from 150 to
250.degree. C., e.g., 160 to 240.degree. C., 170-230.degree. C.
with the temperature range of 180 to 225.degree. C. being a
preferred range. The carbon monoxide partial pressure in the
reactor can vary widely but is typically from 2 to 30 atm, e.g.,
from 3 to 10 atm. The hydrogen partial pressure in the reactor is
typically from 0.05 to 2 atm, e.g., from 1 to 1.9 atm. Some
embodiments of the present invention may be operated with a
hydrogen partial pressure from 0.3 to 2 atm, e.g., from 0.3 to 1.5
atm, or from 0.4 to 1.5 atm. Because of the partial pressure of
by-products and the vapor pressure of the contained liquids, the
total reactor pressure may range from 15 to 40 atm. The production
rate of acetic acid may be from 5 to 50 mol/Lh, e.g., from 10 to 40
mol/Lh, and preferably 15 to 35 mol/Lh. As indicated herein, the
carbonylation reaction is conducted under conditions that do not
produce acetic anhydride.
Exemplary reaction and acetic acid recovery system 100 is shown in
FIG. 1. As shown, methanol-containing feed stream 101 and carbon
monoxide-containing feed stream 102 are directed to liquid phase
carbonylation reactor 104, in which the carbonylation reaction
occurs to form acetic acid.
Carbonylation reactor 104 is preferably either a stirred vessel or
bubble-column type vessel, with or without an agitator, within
which the reacting liquid or slurry contents are maintained,
preferably automatically, a predetermined level, which preferably
remains substantially constant during normal operation. Into
carbonylation reactor 104, fresh methanol, carbon monoxide, and
sufficient water are continuously introduced as needed to maintain
suitable concentrations in the reaction medium.
In a typical carbonylation process, carbon monoxide is continuously
introduced into the carbonylation reactor, desirably below the
agitator, which may be used to stir the contents. The gaseous feed
preferably is thoroughly dispersed through the reacting liquid by
this stirring means. Gaseous purge stream 106 desirably is vented
from the reactor 104 to prevent buildup of gaseous by-products and
to maintain a set carbon monoxide partial pressure at a given total
reactor pressure. In one embodiment, the gaseous purge stream 106
contains low amounts of hydrogen iodide of no more than 1 wt. %,
e.g., no more than 0.9 wt. %, no more than 0.8 wt. %, no more than
0.7 wt. %, no more than 0.5 wt. %, no more than 0.3 wt. %. Hydrogen
iodide in excess of these amounts may increase the duty on the
scrubber to prevent hydrogen iodide from being purged. The
temperature of the reactor may be controlled and the carbon
monoxide feed is introduced at a rate sufficient to maintain the
desired total reactor pressure. Stream 113 comprising the liquid
reaction medium exits reactor 104.
The acetic acid production system preferably includes primary
purification train 108 employed to recover the acetic acid and
recycle catalyst solution, methyl iodide, methyl acetate, and other
system components within the process. Primary purification train
108 include light ends column 124 and drying column 130, and the
associated pumps, overhead receivers, condensers, etc. Thus, a
recycled catalyst solution, such as stream 110 from flash vessel
112, and optionally one or more of recycle streams 114, 116, 118,
and 120, also are introduced into the reactor 104. Of course, one
or more of the recycle streams may be combined prior to being
introduced into the reactor. The separation system also preferably
controls water and acetic acid content in the carbonylation
reactor, as well as throughout the system, and facilitates PRC
removal.
Flash Vessel
The reaction medium is drawn off from the carbonylation reactor 104
at a rate sufficient to maintain a constant level therein and is
provided to flash vessel 112 via stream 113. In flash vessel 112,
the crude product is separated in a flash separation step to obtain
a vapor product stream 122 comprising acetic acid and less volatile
stream 110, e.g., a liquid recycle stream, comprising a
catalyst-containing solution (predominantly acetic acid containing
the rhodium and the iodide salt along with lesser quantities of
methyl acetate, methyl iodide, and water), which preferably is
recycled to the reactor, as discussed above. The vapor product
stream 122 also comprises methyl iodide, methyl acetate, water, and
permanganate reducing compounds (PRC's). Dissolved gases exiting
the reactor and entering the flash vessel comprise a portion of the
carbon monoxide and may also contain gaseous by-products such as
methane, hydrogen, and carbon dioxide. Such dissolved gases exit
the flash vessel as part of the overhead stream.
In one embodiment, vapor product stream 122 comprises acetic acid,
methyl iodide, methyl acetate, water, acetaldehyde, and hydrogen
iodide. In one embodiment, vapor product stream 122 comprises
acetic acid in an amount from 45 to 75 wt. %, methyl iodide in an
amount from 20 to 50 wt. %, methyl acetate in an amount of no more
than 9 wt. %, and water in an amount of no more than 15 wt. %,
based on the total weight of the vapor product stream. In another
embodiment, vapor product stream 122 comprises acetic acid in an
amount from 45 to 75 wt. %, methyl iodide in an amount from 24 to
less than 36 wt. %, methyl acetate in an amount of no more than 9
wt. %, and water in an amount of no more than 15 wt. %, based on
the total weight of the vapor product stream. More preferably,
vapor product stream 122 comprises acetic acid in an amount from 55
to 75 wt. %, methyl iodide in an amount from 24 to 35 wt. %, methyl
acetate in an amount from 0.5 to 8 wt. %, and water in an amount
from 0.5 to 14 wt. %. In yet a further preferred embodiment, vapor
product stream 112 comprises acetic acid in an amount from 60 to 70
wt. %, methyl iodide in an amount from 25 to 35 wt. %, methyl
acetate in an amount from 0.5 to 6.5 wt. %, and water in an amount
from 1 to 8 wt. %. The acetaldehyde concentration in the vapor
product stream may be in an amount from 0.005 to 1 wt. %, based on
the total weight of the vapor product stream, e.g., from 0.01 to
0.8 wt. %, or from 0.01 to 0.7 wt. %. In some embodiments the
acetaldehyde may be present in amounts no more than 0.01 wt. %.
Vapor product stream 122 may comprise hydrogen iodide in an amount
no more than 1 wt. %, based on the total weight of the vapor
product stream, e.g., no more than 0.5 wt. %, or no more than 0.1
wt. %. Vapor product stream 122 is preferably substantially free
of, i.e., contains no more than 0.0001 wt. %, propionic acid, based
on the total weight of the vapor product stream. Similar to the
process stream disclosed herein, vapor product stream 122 is
substantially free of acetic anhydride.
Less volatile stream 110 comprises acetic acid, the metal catalyst,
corrosion metals, as well as other various compounds. In one
embodiment, liquid recycle stream comprises acetic acid in an
amount from 60 to 90 wt. %, metal catalyst in an amount from 0.01
to 0.5 wt. %; corrosion metals (e.g., nickel, iron and chromium) in
a total amount from 10 to 2500 wppm; lithium iodide in an amount
from 5 to 20 wt. %; methyl iodide in an amount from 0.5 to 5 wt. %;
methyl acetate in an amount from 0.1 to 5 wt. %; water in an amount
from 0.1 to 8 wt. %; acetaldehyde in an amount of no more than 1
wt. % (e.g., from 0.0001 to 1 wt. % acetaldehyde); and hydrogen
iodide in an amount of no more than 0.5 wt. % (e.g., from 0.0001 to
0.5 wt. % hydrogen iodide).
Recovery of Acetic Acid
The distillation and recovery of acetic acid is not particularly
limited for the purposes of the present invention. In one
embodiment, the process involves carbonylating, in a reactor, at
least one member selected from the group consisting of methanol,
dimethyl ether, and methyl acetate in a reaction medium comprising
water at a concentration from 0.1 to 14 wt. %, a rhodium catalyst,
methyl iodide, and an iodide salt, separating the reaction medium
to form a liquid recycle stream and a vapor product stream,
distilling at least a portion of the vapor product stream in a
first column to obtain a side stream comprising acetic acid at a
concentration greater than 90 wt. %, water at a concentration from
1 to 3 wt. %, one or more C.sub.1-C.sub.14 alkyl iodides in a total
concentration of no more than 6 wt. % and methyl acetate at a
concentration of no more than 6 wt. %, distilling the side stream
in a second column to obtain a product stream comprising acetic
acid, water at a concentration of no more than 0.2 wt. % and acetic
anhydride at a concentration of no more than 600 wppm, and
contacting the product stream with a cation exchange resin to form
a purified acetic acid product comprising no more than 50 wppm
acetic anhydride. Various embodiments of primary purification train
having up to 2 distillation columns is further described
herein.
First Column
For purposes of the present invention a process stream refers to
any stream that is fed to the distillation column. In one
embodiment vapor product stream 122 may be a process stream. The
overhead stream from flash vessel 112 is directed to the light ends
column 124 (first column) as vapor product stream 122, where
distillation yields a low-boiling overhead vapor stream 126, a
sidedraw 128 that contains acetic acid, and a high boiling residue
stream 116. In one embodiment, vapor product stream 122 may
comprise acetic acid, methyl acetate, water, methyl iodide, and
acetaldehyde, along with other impurities such as hydrogen iodide
and crotonaldehyde, and byproducts such as propionic acid. Acetic
acid removed via sidedraw 128 preferably is subjected to further
purification, such as in drying column 130 (second column) for
selective separation of acetic acid from water as described further
herein.
In one embodiment, sidedraw 128 is removed from a location above
the feed of vapor product stream 122 to light ends column 124.
Thus, acetic anhydride does not form because the water
concentrations in that location of light ends column 124 at greater
than 0.2 wt. %, greater than 0.5 wt. %, or greater than 1 wt.
%.
Light ends column 124 also preferably forms residuum or bottoms
stream 116, which comprises primarily acetic acid and water. Since
light ends bottoms stream 116 typically comprises some residual
catalyst, it may be beneficial to recycle all or a portion of light
ends bottoms stream 116 to reactor 104. Optionally, light ends
bottoms stream 116 may be combined with the catalyst phase 110 from
flash vessel 112 and returned together to reactor 104, as shown in
FIG. 1. Although the concentration of acetic acid may be relatively
high in high boiling residue stream 116, the mass flow of the high
boiling residue stream 116 relative to side stream 128 is very
small. In embodiments, the mass flow of the boiling residue stream
116 is no more than 0.75% of side stream 128, e.g., no more than
0.55%, or no more than 0.45%.
In one embodiment, low-boiling overhead vapor stream 126 comprises
water in amount greater than or equal to 5 wt. %, e.g., greater
than or equal to 10 wt. %, or greater than or equal to 25 wt. %.
The amount of water may be up to 80 wt. %. In terms of ranges,
water concentration in the overhead may be from 5 wt. % to 80 wt.
%, e.g., from 10 wt. % to 70 wt. % or from 25 wt. % to 60 wt. %.
Reducing water concentration to less than 5 wt. % is not
advantageous because this results in a large recycle of acetic acid
back to the reaction system which then sets up a large recycle
through the entire purification system. In addition to water,
low-boiling overhead vapor stream 126 may also comprise methyl
acetate, methyl iodide, and carbonyl impurities, which are
preferably concentrated in the overhead to be removed from acetic
acid in side stream 128. These carbonyl impurities may also be
referred to herein as PRC's.
As shown, low-boiling overhead vapor stream 126 preferably is
condensed and directed to an overhead phase separation unit, as
shown by overhead decanter 134. Conditions are desirably maintained
such that the condensed low-boiling overhead vapor stream 126, once
in decanter 134, may separate to form a light liquid phase 138 and
a heavy liquid phase 118. The phase separation should be maintain
two separate phase, without forming a third phase or emulsion
between the phases. An offgas component may be vented via line 136
from decanter 134. In embodiments, the average residence time of
the condensed low-boiling overhead vapor stream 126 in overhead
decanter 134 is greater than or equal to 1 minute, e.g., greater
than or equal to 3 minutes, greater than or equal to 5 minutes,
greater than or equal to 10 minutes, and/or the average residence
time is no more than 60 minutes, e.g., no more than 45 minutes, or
no more than 30 minutes, or no more than 25 minutes.
Although the specific compositions of the light phase stream 138
may vary widely, some preferred compositions are provided below in
Table 1.
TABLE-US-00001 TABLE 1 Exemplary Light Liquid Phase from Light Ends
Overhead conc. (Wt. %) conc. (Wt. %) conc. (Wt. %) HOAc 1-40 1-25
5-15 Water 50-90 50-80 60-80 PRC's <5 <3 <1 MeI <10
<5 <3 MeAc 1-50 1-25 1-15
In one embodiment, overhead decanter 134 is arranged and
constructed to maintain a low interface level to prevent an excess
hold up of methyl iodide. Although the specific compositions of
heavy liquid phase 118 may vary widely, some exemplary compositions
are provided below in Table 2.
TABLE-US-00002 TABLE 2 Exemplary Heavy Liquid Phase from Light Ends
Overhead conc. (Wt. %) conc. (Wt. %) conc. (Wt. %) Water 0.01-2
0.05-1 0.1-0.9 Methyl Acetate 0.1-25 0.5-20 0.7-15 Acetic Acid
0.1-10 0.2-8 0.5-6 PRC's <5 <3 <1 Methyl Iodide 40-98
50-95 60-85
The density of the heavy liquid phase 118 may be from 1.3 to 2,
e.g., from 1.5 to 1.8, from 1.5 to 1.75 or from 1.55 to 1.7. As
described in U.S. Pat. No. 6,677,480, the measured density in the
heavy liquid phase 118 correlates with the methyl acetate
concentration in the reaction medium. As density decreases, the
methyl acetate concentration in the reaction medium increases. In
one embodiment of the present invention heavy liquid phase 118 is
recycled to the reactor and the light liquid phase 138 is
controlled to be recycled through the same pump. It may be
desirable to recycle a portion of the light liquid phase 138 that
does not disrupt the pump and maintains a density of the combined
light liquid phase 138 and heavy liquid phase of greater than or
equal to 1.3, e.g., greater than or equal to 1.4, greater than or
equal to 1.5, or greater than or equal to 1.7. As described herein,
a portion of the heavy liquid phase 118 may be treated to remove
impurities such as acetaldehyde.
As shown in FIG. 1, the light phase exits decanter 134 via stream
138. A first portion, e.g., aliquot portion, of light phase stream
138 is recycled to the top of the light ends column 124 as reflux
stream 140. In other embodiments a portion of the heavy liquid
phase 118 may also be refluxed (not shown) to the light ends column
124.
PRC Removal System
As described herein the light ends column 124 is part of the
primary purification train. In some embodiments, a portion of light
liquid phase and/or heavy liquid phase may be separated and
directed to acetaldehyde or PRC removal system 132 to recover
methyl iodide and methyl acetate, while removing acetaldehyde. For
purposes of the present invention, the acetaldehyde or PRC removal
system 132 is not part of the primary purification train.
As shown in Tables 1 and 2, light liquid phase 133 and/or heavy
liquid phase 118 each contain PRC's and the process may include
removing carbonyl impurities, such as acetaldehyde, that
deteriorate the quality of the acetic acid product and may be
removed in suitable impurity removal columns and absorbers as
described in U.S. Pat. Nos. 6,143,930; 6,339,171; 7,223,883;
7,223,886; 7,855,306; 7,884,237; 8,889,904; and US Pub. Nos.
2006/0011462, which are incorporated herein by reference in their
entirety. Carbonyl impurities, such as acetaldehyde, may react with
iodide catalyst promoters to form alkyl iodides, e.g., ethyl
iodide, propyl iodide, butyl iodide, pentyl iodide, hexyl iodide,
etc. Also, because many impurities originate with acetaldehyde, it
is desirable to remove carbonyl impurities from the liquid light
phase.
The portion of light liquid phase 138 and/or heavy liquid phase 118
fed to the acetaldehyde or PRC removal system 132 via lines 142 and
143, respectively, may vary from 1% to 99% of the mass flow of
either the light liquid phase 138 and/or heavy liquid phase 118,
e.g., from 1 to 50%, from 2 to 45%, from 5 to 40%, 5 to 30% or 5 to
20%. Also in some embodiments, a portion of both the light liquid
phase 138 and heavy liquid phase 118 may be fed to the acetaldehyde
or PRC removal system 132. The portion of the light liquid phase
138 not fed to the acetaldehyde or PRC removal system 132 may be
refluxed to the first column or recycled to the reactor, as
described herein. The portion of the heavy liquid phase 118 not fed
to the acetaldehyde or PRC removal system 132 may be recycled to
the reactor. Although a portion of heavy liquid phase 118 may be
refluxed to the light ends column, it is more desirable to return
the methyl iodide enriched heavy liquid phase 118 to the
reactor.
In one embodiment, a portion of light liquid phase 138 and/or heavy
liquid phase 118 is fed to a distillation column which enriches the
overhead thereof to have acetaldehyde and methyl iodide. Depending
on the configuration, there may be two separate distillation
columns, and the overhead of the second column may be enriched in
acetaldehyde and methyl iodide. Dimethyl ether, which may be formed
in-situ, may also be present in the overhead. The overhead may be
subject to one or more extraction stages to remove a raffinate
enriched in methyl iodide and an extractant. A portion of the
raffinate may be returned to the distillation column, first column,
overhead decanter and/or reactor. For example, when the heavy
liquid phase 118 is treated in the PRC removal system 132, it may
be desirable to return a portion the raffinate to either the
distillation column or reactor. Also, for example, when light
liquid phase 138 is treated in the PRC removal system 132, it may
be desirable to return a portion the raffinate to either the first
column, overhead decanter, or reactor. In some embodiments, the
extractant may be further distilled to remove water, which is
returned to the one or more extraction stages. The column bottoms,
which contains more methyl acetate and methyl iodide than light
liquid phase 138, may also be recycled to reactor 104 and/or
refluxed to light ends column 124.
Drying Column
Returning to the primary purification train, in addition to the
overhead phase, the light ends column 124 also forms an acetic acid
sidedraw 128, which preferably comprises primarily acetic acid and
water, and is substantially free of acetic anhydride. In one
embodiment, acetic acid sidedraw 128 is a process stream. In one
embodiment, acetic acid sidedraw 128 comprises acetic acid in
amount of greater than or equal to 90 wt. %, e.g., greater than or
equal to 94 wt. % or greater than or equal to 96 wt. %. The water
concentration of the process stream may be in an amount from 1 to 3
wt. %, e.g., from 1 to 2.5 wt. % and more preferably from 1.1 to
2.1 wt. %. The acetic acid sidedraw 128 may also comprise one or
more C.sub.1-C.sub.14 alkyl iodides in a total concentration of no
more than 6 wt. %, e.g., no more than 4 wt. %, or no more than 3.6
wt. %, and methyl acetate at a concentration of no more than 6 wt.
%, e.g., no more than 4 wt. %, or no more than 3.6 wt. %. In some
embodiments, in addition to acetic acid and water, acetic acid
sidedraw 128 may also comprise one or more C.sub.1-C.sub.14 alkyl
iodides in an amount from 0.1 to 6 wt. %, e.g., from 0.5 to 5 wt.
%, from 0.6 to 4 wt. %, from 0.7 to 3.7 wt. %, or from 0.8 to 3.6
wt. %. Due to the presence of water, acetic acid sidedraw 128 may
also contain methyl acetate in an amount from 0.1 to 6 wt. %, e.g.,
from 0.5 to 5 wt. %, from 0.6 to 4 wt. %, from 0.7 to 3.7 wt. %, or
from 0.8 to 3.6 wt. %. In some embodiments, acetic acid sidedraw
128 may also comprise hydrogen iodide at a concentration of no more
than 300 wppm, e.g., or no more than 250 wppm, no more than 200
wppm, no more than 100 wppm, no more than 50 wppm, no more than 25
wppm, or no more than 10 wppm.
In one embodiment, to maintain an efficient product separation, it
is highly desired that the composition of the sidedraw 128 does not
vary or fluctuate significantly during normal operation. By does
not vary or fluctuate significantly it is meant that the
concentration of the one or more C.sub.1-C.sub.14 alkyl iodides and
the concentration of methyl acetate is .+-.0.9% of the water
concentration in the side stream, e.g., .+-.0.7%, .+-.0.5%,
.+-.0.4%, .+-.0.3%, .+-.0.2%, or .+-.0.1%. The water concentration
in the side stream may be from 1 to 3 wt. %, e.g., preferably from
1.1 to 2.5 wt. %. For example, when the water concentration is 2.5
wt. %, the concentration of C.sub.1-C.sub.14 alkyl iodides is from
1.6 to 3.4 wt. %, and the concentration of methyl acetate is from
1.6 to 3.4 wt. %.
Optionally, a portion of the sidedraw 128 may be recirculated to
the light ends column, preferably to a point below where sidedraw
128 was removed from light ends column, in order to improve the
separation (not shown).
Since sidedraw 128 contains water in addition to acetic acid,
sidedraw 128 from the light ends column 124 preferably is directed
to drying column 130, in which the acetic acid and water are
separated from one another. As shown, drying column 130, separates
acetic acid sidedraw 128 to form overhead stream 144 comprised
primarily of water and a product stream 146 comprised primarily of
acetic acid. Overhead stream 144 preferably is cooled and condensed
in a phase separation unit, e.g., decanter 148, to form a light
phase 150 and a heavy phase 122. As shown, a portion of the light
phase is refluxed, as shown by stream 152 and the remainder of the
light phase is returned to the reactor 104, as shown by stream 120.
The heavy phase, which typically is an emulsion comprising water
and methyl iodide, preferably is returned in its entirety to the
reactor 104, as shown by stream 122, optionally after being
combined with stream 120.
Exemplary compositions for the light phase of the drying column
overhead are provided below in Table 3.
TABLE-US-00003 TABLE 3 Exemplary Light Phase Compositions from
Drying Column Overhead conc. (Wt. %) conc. (Wt. %) conc. (Wt. %)
HOAc 1-20 1-15 1-10 Water 50-90 60-90 70-90 MeI <10 <5 <3
MeAc 1-20 1-15 1-10
In certain embodiments, as discussed, minor amounts of an alkali
component such as KOH can be added to sidedraw 128 via line 160
prior to entering the drying column 130. In other embodiments, the
alkali component might also be added to the drying column 130 at
the same height level as the stream 128 entering the drying column
130 or at a height above the height level height level as the
stream 128 entering the drying column 130. Such addition can
neutralize HI in the column.
Product stream 146 preferably comprises or consists essentially of
acetic acid. In further embodiments, it is preferred not to dilute
product stream 146 with an aqueous diluent, such as water. In one
embodiment, product stream 146 comprises acetic acid at a
concentration of greater than or equal to 99.5 wt. %, e.g., greater
than or equal to 99.7 wt. % or greater than or equal to 99.9 wt. %.
Product stream 146 comprises water at a concentration of no more
than 0.2 wt. %, e.g., no more than 0.15 wt. %, no more than 0.1 wt.
%, or no more than 0.05 wt. %. Due to anhydrous conditions in
drying column 130, acetic anhydride may be formed. In one
embodiment, product stream 146 comprises acetic anhydride at a
concentration of no more than 600 wppm, e.g., no more than 500
wppm, no more than 450 wppm, no more than 400 wppm, no more than
300 wppm, no more than 200 wppm, no more than 100 wppm, or no more
than 50 wppm. In terms of ranges, product stream 146 comprises
acetic anhydride in an amount from 5 to 600 wppm, e.g., from 5 to
500 wppm, from 5 to 450 wppm, from 10 to 450 wppm, from 10 to 300
wppm, or from 10 to 100 wppm.
In some embodiments, in addition to acetic anhydride, product
stream 146 may also comprise lithium in an amount of up to or equal
to 10 wppm, e.g., up to or equal to 5 wppm, up to or equal to 1
wppm, up to or equal to 500 wppb, up to or equal to 300 wppb, or up
to or equal to 100 wppb.
Thus, in one embodiment, there is provided a process for producing
acetic acid comprising separating a reaction medium formed in a
reactor in a flash vessel to form a liquid recycle and a vapor
product stream, distilling the vapor product stream in a first
column to obtain a side stream and a low boiling overhead vapor
stream comprising water in an amount of greater than or equal to 5
wt. %, condensing the low boiling overhead vapor stream and
biphasically separating the condensed stream to form a heavy liquid
phase and a light liquid phase, optionally treating a portion of
the heavy liquid phase and/or the light liquid phase to remove at
least one PRC, distilling the side stream in a second column to
obtain a product stream comprising acetic acid, water at a
concentration of no more than 0.2 wt. % and acetic anhydride at a
concentration of no more than 600 wppm, and contacting the product
stream with a cation exchange resin to form a purified acetic acid
product comprising no more than 50 wppm acetic anhydride.
As shown in FIG. 2, in certain embodiments, the product stream
withdrawn from the drying column 130 may be taken from a sidedraw
170 at a position slightly above the bottom 172 of the column 130.
In one embodiment, sidedraw 170 is withdrawn within 5 actual stages
from the bottom 172 of the column 130, e.g., within 4 actual stages
from the bottom of the column 130, within 3 actual stages from the
bottom of the column 130, or within 2 actual stages from the bottom
of the column 130. In some embodiments, the sidedraw 170 is
withdrawn a position between 2 and 5 actual stages from the bottom
172 of the column 130, e.g., a position between 3 and 5 trays from
the bottom of the column 130, or position between 3 and 4 trays
from the bottom of the column 130. In one embodiment, the product
stream may be in sidedraw 170 that is withdrawn in the liquid phase
so that the lithium cation concentration would be similar to the
withdrawn lithium cation concentration in the drying columns
bottoms in stream 146. In other embodiments, sidedraw 170 may be a
vapor stream and the lithium cation concentration may be less than
the bottoms stream 146. When in the vapor phase, it is desirable to
condense sidedraw 170 prior to contacting the cationic exchanger
resin. When a sidedraw 170 is used then other impurities such as
heavy carbonyl containing groups, i.e. propionic acid, may
advantageously concentrate in the bottoms stream 174. Residue
stream 174 may be discarded or purged from the process 100.
Sidedraw 170 contains the product stream that is contacted with the
cationic exchange resin to hydrate acetic anhydride according to
embodiments of the present invention. In one embodiment there is
provided a process for producing acetic acid comprising separating
a reaction medium formed in a reactor in a flash vessel to form a
liquid recycle and a vapor product stream, distilling the vapor
product stream in a first column to obtain a side stream and a low
boiling overhead vapor stream comprising water in an amount of
greater than or equal to 5 wt. %, condensing the low boiling
overhead vapor stream and biphasically separating the condensed
stream to form a heavy liquid phase and a light liquid phase,
optionally treating a portion of the heavy liquid phase and/or the
light liquid phase to remove at least one PRC, distilling the side
stream in a second column to obtain a sidedraw, either in the
liquid or vapor phase, wherein the sidedraw comprises acetic acid,
water at a concentration of no more than 0.2 wt. % and acetic
anhydride at a concentration of no more than 600 wppm, and
contacting the sidedraw, or a condensed portion thereof, with a
cation exchange resin to form a purified acetic acid product
comprising no more than 50 wppm acetic anhydride.
In some embodiments, the product stream withdrawn from the bottoms
or the sidedraw may also be processed to remove lithium derived
from and/or generated by the lithium compound in the reaction
medium, by passing through cationic exchanger in the acid form and
then through metal functionalized iodide removal ion exchange
resins, prior to being stored or transported for commercial use. As
described herein, cationic exchanger in the acid form are suitable
for removing cations, such as lithium derived from and/or generated
by compounds in the reaction medium that concentrate in the crude
acid product. Once these components, and in particular lithium, are
removed the iodides may be removed by metal functionalized iodide
removal ion exchange resins.
Iodide Removal Beds/Use of Ion Exchange Resins
According to the present process, product stream that are
contaminated with acetic anhydride and halides (e.g., iodides) may
be contacted with an acid-form cationic exchange resin to hydrate
the acetic anhydride followed by a metal-exchanged ion exchange
resin having acid cation exchange sites comprising at least one
metal selected from the group consisting of silver, mercury,
palladium and rhodium under a range of operating conditions.
Preferably, the ion exchange resin compositions are provided in
fixed beds. The use of fixed iodide removal beds to purify
contaminated carboxylic acid streams is well documented in the art
(see, for example, U.S. Pat. Nos. 4,615,806; 5,653,853; 5,731,252;
and 6,225,498, which are hereby incorporated by reference in their
entireties). Generally, a contaminated liquid carboxylic acid
stream is contacted with the aforementioned ion exchange resin
compositions, by flowing through a series of static fixed beds. In
one embodiment, the cationic exchange resin may be used to hydrate
at least 60% of the acetic anhydride in the product stream to yield
a purified acetic acid product having acetic anhydride at a
concentration of no more than 50 wppm. In some embodiments,
cations, such as lithium contaminants may also be removed by the
cationic exchange resin in the acid form. The halide contaminants,
e.g., iodide contaminants, are then removed by reaction with the
metal of the metal-exchanged ion exchange resin to form metal
iodides. In some embodiments, hydrocarbon moieties, e.g., methyl
groups, that may be associated with the iodide may esterify the
carboxylic acid. For example, in the case of acetic acid
contaminated with methyl iodide, methyl acetate would be produced
as a byproduct of the iodide removal. The formation of this
esterification product typically does not have a deleterious effect
on the treated carboxylic acid stream.
Similar iodide contamination issues may exist in acetic anhydride
manufactured via a rhodium-iodide catalyst system. Thus, the
inventive process may alternatively be utilized in the purification
of crude acetic anhydride product streams.
Suitably stable ion exchange resins utilized in connection with the
present invention for preparing silver or mercury-exchanged resins
for iodide removal typically are of the "RSO.sub.3H" type
classified as "strong acid," that is, sulfonic acid, cation
exchange resins of the macroreticular (macroporous) type.
Particularly suitable ion exchange substrates include
Amberlyst.RTM. 15, Amberlyst.RTM. 35 and Amberlyst.RTM. 36 resins
(DOW) suitable for use at elevated temperatures. Other stable ion
exchange substrates such as zeolites may be employed, provided that
the material is stable in the organic medium at the conditions of
interest, that is, will not chemically decompose or release silver
or mercury into the organic medium in unacceptable amounts. Zeolite
cationic exchange substrates are disclosed, for example, in U.S.
Pat. No. 5,962,735, the disclosure of which is incorporated herein
by reference.
At temperatures greater than about 50.degree. C., the silver or
mercury exchanged cationic substrate may tend to release small
amounts of silver or mercury on the order of 500 wppb or less and
thus the silver or mercury exchanged substrate is chemically stable
under the conditions of interest. More preferably, silver losses
are less than 100 wppb into the organic medium and still more
preferably less than 20 wppb into the organic medium. Silver losses
may be slightly higher upon start up. In any event, if so desired a
bed of acid form cationic material may be placed downstream of the
silver or mercury exchange material in addition to the bed of acid
form cationic material upstream of the silver or mercury exchange
material, to catch any silver or mercury released.
The pressures during the contacting steps with the exchange resins
are limited only by the physical strength of the resins. In one
embodiment, the contacting is conducted at pressures ranging from
0.1 MPa to 1 MPa, e.g., from 0.1 MPa to 0.8 MPa or from 0.1 MPa to
0.5 MPa. For convenience, however, both pressure and temperature
preferably may be established so that the contaminated carboxylic
acid stream is processed as a liquid. Thus, for example, when
operating at atmospheric pressure, which is generally preferred
based on economic considerations, the temperature may range from
17.degree. C. (the freezing point of acetic acid) to 118.degree. C.
(the boiling point of acetic acid). It is within the purview of
those skilled in the art to determine analogous ranges for product
streams comprising other carboxylic acid compounds. The temperature
of the contacting step preferably is kept low enough to minimize
resin degradation. In one embodiment, the contacting is conducted
at a temperature ranging from 25.degree. C. to 120.degree. C.,
e.g., from 25.degree. C. to 100.degree. C. or from 50.degree. C. to
100.degree. C. Some cationic macroreticular resins typically begin
significant degrading (via the mechanism of acid-catalyzed aromatic
desulfonation) at temperatures of 150.degree. C. Carboxylic acids
having up to 5 carbon atoms, e.g., up to 4 carbon atoms, or up to 3
carbon atoms, remain liquid at these temperatures. Thus, the
temperature during the contacting should be maintained below the
degradation temperature of the resin utilized. In some embodiments,
the operating temperature is kept below temperature limit of the
resin, consistent with liquid phase operation and the desired
kinetics for lithium and/or halide removal.
The configuration of the resin beds within an acetic acid
purification train may vary, but the cationic exchanger should be
upstream of the metal-exchanged resin. In a preferred embodiment,
the resin beds are configured after a drying column. Preferably the
resin beds are configured in a position wherein the temperature of
the product stream is low, e.g., less than 120.degree. C. or less
than 100.degree. C. The stream contacting the acid-form cationic
exchange resin and the stream contacting the metal-exchanged resin
can be adjusted to the same or different temperatures. For example,
the stream contacting the acid-form cationic exchange resin can be
adjusted to a temperature from 25.degree. C. to 120.degree. C.,
e.g., 30.degree. C. to 100.degree. C., 25.degree. C. to 85.degree.
C., 40.degree. C. to 70.degree. C., e.g., 40.degree. C. to
60.degree. C., while the stream contacting the metal-exchanged
resin can be adjusted to a temperature from 50.degree. C. to
100.degree. C., e.g., from 50.degree. C. to 85.degree. C., from
55.degree. C. to 75.degree. C., or from 60.degree. C. to 70.degree.
C. Aside from the advantages discussed above, lower temperature
operation provides for less corrosion as compared to higher
temperature operation. Lower temperature operation provides for
less formation of corrosion metal contaminants, which, as discussed
above, may decrease overall resin life. Also, because lower
operating temperatures result in less corrosion, vessels
advantageously need not be made from expensive corrosion-resistant
metals, and lower grade metals, e.g., standard stainless steel, may
be used.
Referring back to FIG. 1, product stream 146 is first passed
through cationic exchange resin bed 180 to hydrate acetic
anhydride. Although one cationic exchange resin bed 180 is shown,
it should be understood that a plurality of cationic exchange resin
beds may be used in series or parallel. In some embodiments, the
cationic exchangers may also remove other cations present in the
stream, such as lithium or potassium, if added via line 160 to
drying column 130 as a potassium salt selected from the group
consisting of potassium acetate, potassium carbonate, and potassium
hydroxide, and corrosion metals. Using the cationic exchangers of
the present invention, the purified acetic acid product comprises
less acetic anhydride than the product stream.
The resulting purified acetic acid 182 may passes through a
metal-exchanged ion exchange resin bed 184 having acid cation
exchange sites comprising at least one metal selected from the
group consisting of silver, mercury, palladium and rhodium to
remove iodides from the stream to produce a purified product 186.
Although one metal-exchanged ion exchange resin bed 184 is shown,
it should be understood that a plurality of metal-exchanged ion
exchange resin beds may be used in series or parallel. In addition
to the resin beds, heat exchangers (not shown) may be located
before either resin bed to adjust the temperature of the stream 146
and 182 to the appropriate temperature before contacting the resin
beds. Similarly in FIG. 2, the crude acetic acid product is fed to
cationic exchange resin bed 180 from side stream 170. Heat
exchangers or condensers may be located before either resin bed to
adjust the temperature of the stream 170 to the appropriate
temperature before contacting the resin beds.
In one embodiment, the flow rate through the resin beds ranges from
0.1 bed volumes per hour ("BV/hr") to 50 BV/hr, e.g., 1 BV/hr to 20
BV/hr or from 6 BV/hr to 10 BV/hr. A bed volume of organic medium
is a volume of the medium equal to the volume occupied by the resin
bed. A flow rate of 1 BV/hr means that a quantity of organic liquid
equal to the volume occupied by the resin bed passes through the
resin bed in a one hour time period.
A purified acetic acid composition is obtained as a result of the
resin bed treatment. The purified acetic acid composition comprises
acetic anhydride at a concentration of no more than 50 wppm, e.g.,
no more than 40 wppm, no more than 30 wppm, no more than 20 wppm,
no more than 10 wppm or no more than 5 wppm. In terms of ranges,
the purified acetic acid product comprises acetic anhydride at a
concentration from 0.5 to 50 wppm, e.g., from 0.5 to 40 wppm, from
0.5 to 30 wppm, from 0.5 to 20 wppm, or from 0.5 to 10 wppm. The
purified acetic acid product comprises less acetic anhydride than
the product stream. The purified acetic acid composition, in one
embodiment, comprises iodides in an amount of no more than 100
wppb, e.g., no more than 90 wppb, no more than 50 wppb, no more
than 25 wppb, or no more than 15 wppb. In one embodiment, the
purified acetic acid composition comprises lithium in an amount of
no more than 100 wppb, e.g., no more than 50 wppb, no more than 20
wppb, or no more than 10 wppb. In terms of ranges, the purified
acetic acid composition may comprise from 0 to 100 wppb iodides,
e.g., from 0 to 50 wppb, from 1 to 50 wppb, from 2 to 40 wppb;
and/or from 0 to 100 wppb lithium, e.g., from 1 to 50 wppb, from 2
to 40 wppb. In other embodiments, the resin beds remove at least 25
wt. % of the iodides from the product stream, e.g., at least 50 wt.
% or at least 75 wt. %.
In addition to reducing acetic anhydride, the present invention may
also reduce a metal displaced from the metal-exchanged ion exchange
resin, e.g. silver, mercury, palladium and rhodium, that
undesirably accumulate in the purified acetic acid as the final
product when no cationic exchanger is used to remove cations, such
as lithium derived from and/or generated by the lithium compound in
the reaction medium. In one embodiment, the purified acetic acid
comprises a metal displaced from the metal-exchanged ion exchange
resin, e.g., silver, mercury, palladium and rhodium, in an amount
of no more than 100 wppb, e.g., no more than 90 wppb, no more than
80 wppb, no more than 70 wppb, no more than 60 wppb, no more than
50 wppb, no more than 40 wppb, no more than 30 wppb, or no more
than 20 wppb. In terms of ranges, the purified acetic acid
comprises a metal displaced from the metal-exchanged ion exchange
resin, e.g., silver, mercury, palladium and rhodium, in an amount
from 0 to 100 wppb, e.g., from 0.1 to 100 wppb, from 0.5 to 90
wppb, from 1 to 80 wppb, from 1 to 70 wppb, from 1 to 60 wppb, from
1 to 50 wppb, from 1 to 40 wppb, from 1 to 30 wppb, or from 1 to 20
wppb.
Distillation
The distillation columns of the present invention may be a
conventional distillation column, e.g., a plate column, a packed
column, and others. Plate columns may include a perforated plate
column, bubble-cap column, Kittel tray column, uniflux tray, or a
ripple tray column. For a plate column, the theoretical number of
plates is not particularly limited and depending on the species of
the component to be separate, may include up to 80 plates, e.g.,
from 2 to 80, from 5 to 60, from 5 to 50, or more preferably from 7
to 35. The distillation column may include a combination of
different distillation apparatuses. For example, a combination of
bubble-cap column and perforated plate column may be used as well
as a combination of perforated plate column and a packed
column.
The distillation temperature and pressure in the distillation
system can suitably be selected depending on the condition such as
the species of the objective carboxylic acid and the species of the
distillation column, or the removal target selected from the lower
boiling point impurity and the higher boiling point impurity
according to the composition of the feed stream. For example, in a
case where the purification of acetic acid is carried out by the
distillation column, the inner pressure of the distillation column
(usually, the pressure of the column top) may be from 0.01 to 1
MPa, e.g., from 0.02 to 0.7 MPa, and more preferably from 0.05 to
0.5 MPa in terms of gauge pressure. Moreover, the distillation
temperature for the distillation column, namely the inner
temperature of the column at the temperature of the column top, can
be controlled by adjusting the inner pressure of the column, and,
for example, may be from 20 to 200.degree. C., e.g., from 50 to
180.degree. C., and more preferably from 100 to 160.degree. C.
The material of each member or unit associated with the
distillation system, including the columns, valves, condensers,
receivers, pumps, reboilers, and internals, and various lines, each
communicating to the distillation system may be made of suitable
materials such as glass, metal, ceramic, or combinations thereof,
and is not particularly limited to a specific one. According to the
present invention, the material of the foregoing distillation
system and various lines are a transition metal or a
transition-metal-based alloy such as iron alloy, e.g., a stainless
steel, nickel or nickel alloy, zirconium or zirconium alloy
thereof, titanium or titanium alloy thereof, or aluminum alloy.
Suitable iron-based alloys include those containing iron as a main
component, e.g., a stainless steel that also comprises chromium,
nickel, molybdenum and others. Suitable nickel-based alloys include
those alloys containing nickel as a main component and one or more
of chromium, iron, cobalt, molybdenum, tungsten, manganese, and
others, e.g., HASTELLOY.TM. and INCONEL.TM.. Corrosion-resistant
metals may be particularly suitable as materials for the
distillation system and various lines.
As is evident from the figures and text presented above, a variety
of embodiments are contemplated.
The present invention will be better understood in view of the
following non-limiting examples.
EXAMPLES
Example 1
A stream comprising acetic acid, less than 2 wt. % water, and
acetic anhydride that varied from 123 to 510 ppm was fed to a
column filled with Ag functionalized sulfonic resin at 8 bed volume
per hour flow rate. The temperature of the resin was controlled at
75.degree. C. The acetic anhydride concentration in the inlet and
outlet of the resin column were measured by a gas chromatograph
(GC) equipped with a flame ionization detector. A capillary column
with dimethylpolysiloxane stationary phase was used to achieve
separation in the GC analysis. Results for experiments 1-3 are
shown in Table 4.
TABLE-US-00004 TABLE 4 Flow Through Experiments Performed at
75.degree. C. Experiment No. Inlet Conc., ppm Outlet Conc., ppm 1
123 2 2 235 2 3 510 1
Example 2
Experiments 4-6 were performed with the same procedures and setup
as Example 1 except the initial acetic anhydride concentration
varied from 171 to 574 ppm and the resin column temperature that
was controlled at 25.degree. C. Results are shown in Table 5.
TABLE-US-00005 TABLE 5 Flow Through Experiments Performed at
25.degree. C. Experiment No. Inlet Conc., ppm Outlet Conc., ppm 4
171 1 5 333 1 6 574 2
While the invention has been described in detail, modifications
within the spirit and scope of the invention will be readily
apparent to those of skill in the art. In view of the foregoing
discussion, relevant knowledge in the art and references discussed
above in connection with the Background and Detailed Description,
the disclosures of which are all incorporated herein by reference.
In addition, it should be understood that aspects of the invention
and portions of various embodiments and various features recited
below and/or in the appended claims may be combined or interchanged
either in whole or in part. In the foregoing descriptions of the
various embodiments, those embodiments which refer to another
embodiment may be appropriately combined with other embodiments as
will be appreciated by one of skill in the art. Furthermore, those
of ordinary skill in the art will appreciate that the foregoing
description is by way of example only, and is not intended to limit
the invention.
* * * * *